Minnesota's New DUI Law Legality/Grandfather Clause-ish Type Stuff

The obvious notation I have to put in here is that I believe that people that drive under the influence are just really naughty people that have to be punished. In these days of comfortable shoes, I don’t understand why people do it over and over again (much less once). (And I’ve never gotten a Dui/dwi or parking ticket)
Nevertheless, Minnesota’s new law regarding drunk driving began on August 1st. It states that after one’s third offense it is considered a felony (from a misdemeanor/gross misdemeanor). The thing that gets me is that is inclusive of all prior offenses. So if I were to go out and booze it up and drive I’d have to get caught 3 times to get a felony. Whereas if a person who already had 5 DUIs for instance got caught after August 1st, it would be an automatic felony.
To make a long post short, aren’t people “grandfathered in” by law? Or can laws include previous convictions like that?
Here’s a link about the new law in the local paper:

http://www.startribune.com/stories/462/3145907.html

Why would it be fair for somebody to get 8 DUI’s before he spends a significant time in jail when somebody else only gets 3 DUI’s before they are a felon? Just because the law has changed doesn’t mean that the crime is now more dangerous than it was in the past, the person with 5 previous DUIs should hopefully know that.

As an aside, I remember sitting in a resaurant in Tonka Bay and hearing some guy (20 something) fondly (?) recalling his first DUI with some friends. That ain’t right.