I am a firm believer that pot should be legal for a variety are reasons. There is a good chance this will eventually happen. This question isn’t about that so much as what will happen to all the people in prison? I assume they will just serve out their terms right? Could we put that many people back on the streets that fast without causing all sorts of other problems? Unemployment is already pretty high and releasing people wouold just add to that. What are the arguments for what to do with the people who are already in prision for drug offenses
The general principle is that it was illegal at the time, they broke the law.
That’s not to say there might not be pressure to release them but that’s a separate matter.
Do we have any ballpark figure for the number of people who are in prison just for pot?
When booze was legal again in 1933 did they let people out of prison who were in for making booze? My guess is no.
Releasing them will save the cost of keeping them under guard, and their being released won’t change their employment situation. The unemployment rate figures are not really a concern, because it’s just statistical numbers that will shift and not gross national product.
They still broke the law, not to mention a lot of drug offenders have multiple convictions like identify theft, carrying false IDs, using illegal pesticides, selling to minors, resisting arrest, illegal possession of firearms, selling other drugs at the same time, etc. Yes, there are non-violent offenders in jail for growing pot, but thats a rare exception. I dont have much sympathy for them and they will simply serve out the rest of their terms. Why should they be rewarded for breaking the law while the rest of us work hard and pay taxes?
Stoners? Really? 
Not many people actually serve time for mere possession. Usually it’s a fine or community service. As often as not you get a warning and the cop just steals your stash and goes and smokes it with his buddies.
Not to mention tax evasion, federal and otherwise.
Not users so much as dealers. A simple user rarely goes to prison. Dealers can be well armed, and violent.
To answer the Op,most will have their sentences commuted or even be pardoned. But some may have associated charges (such as firearms charges) and will stay in prison.
On the other hand, that would probably reduce the number of people involved in prison labor and would probably put upward pressure on the wages of workers who are no longer competing with people working for prison wages.
Generally in these situations, the political impetus that drives decriminalising the relevant activity also drives letting out any imprisoned offenders. It is not automatically so however, and much, as always, depends on the detail. As an example of what can occur, in my jurisdiction for a short period, trafficking in drugs attracted a mandatory life term. When they got rid of the mandatory life term, at the same time they set up a process to resentence those people who had already been stiffed with life under the older rule, otherwise they would have simply sat and rotted.
Back to the detail, however, a problem for your proposal is this: What do you mean by making pot “legal”? Legal to possess? What about sell? Given that legalisation of possession will almost certainly involve taxation issues (as with alcohol and cigarettes), it is hardly likely that it will ever be legal to sell in the casual way it is sold now. For similar reasons, it is hardly likely ever to be legal to grow it in the casual way it is now. There will be licences, fees, tax stamps on packaging and all the usual rigmarole.
So what do you do with the people who were imprisoned for sale or production when the justification for imprisonment was that “Drugs are EVIL”, once the sale or production is still illegal, but the justification is now “Drugs are a Tax Income stream that needs to be protected”? These things have to be thought through before any proposal to change the rules is undertaken. That is not to say that your primary point might not be correct - possession of pot might, to some minds, be a good thing to legalise. But history can’t be undone as simply as that.
Where does this happen?
I think it’s illegal to run a marijuana thread on this message board without reference to the Netherlands, so allow me to save us all from prosecution.
The Dutch, of course, operate a system whereby marijuana use, possession, and sale is not legal but is instead tolerated under strict guidelines.
Link.
I believe also that growing a couple of plants in your back garden won’t get you into trouble whereas large scale production will attract some negative attention from the police. It is this mass growing of marijuana and supply to the 700+ coffee shops in the Netherlands that may cause a problem for less liberal-minded governments. In effect, the Dutch allow the sale of a product that is the direct result of criminal activity.
The aforementioned coffee shops are taxed on sales of marijuana at the rate of 52%. This is not a value added tax but an income tax. As a basis for taxation, the Dutch equivalent of the IRS makes an assumption that the sale price is twice the purchase price. Therefore it is possible to place casually sold marijuana under a taxation regime, if you see the coffee shop system as casual, but a blind eye must be turned regarding the provenance of the drug if the current system in the Netherlands is to be emulated elsewhere.
I note from the link that state-controlled farming is being planned by the Dutch but I am unaware of any progress in that area.
IIRC, there are some prisons where prisoners work as a call center for commercial businesses, doing telemarketing or customer support.
I’m not sure how many serious stoners qualify for those jobs, though, because they probably talk too slowly. 
Do you guys think that the people who are imprisoned for possessing rifles banned under the assault weapons ban should be let go now that that ban has been lifted?
I feel compelled to point out that vast numbers of weed smokers also work hard and pay their taxes.
It happens in California. They make a damn fine pair of work boots.
That was not HorseLoverFat’s argument. He’d agree with you, and point out that the vast number of weed smokers are not in jail. Some of them get elected as President. The ones in jail, on the other hand, are not just occasional smokers, but growers/dealers who often have a host of other charges.
You’re right, I should have said, “There are large numbers of nonviolent weed growers who also work hard and pay their taxes.”