That others have a different perspective on the question really shouldn’t cause you to have such a visceral reaction.
The problem is that the question is worded ambiguously(which is kinda the entire point of it being a “riddle” or really a trick question). If the question is how much was stolen from the store, then the answer is $100. But that’s not the question, the question is how much did the store lose.
We are given the information that the two interactions that this store has ever had is a theft of $100, and a purchase of $70. If you are asking how much money did the store lose on those two interactions, then the answer is $100 minus the profits of the sale.
Let’s say that I stole $100 from the store. Then you, completely unrelated to me, come in and buy something for $70, giving them a completely different $100 bill, and get $30 in change. What is the store’s loss over those two interactions? If we are the only two people that come into the store that day, what is the store’s loss for that day?
The way that the question is worded only works to give the amount stolen being the amount lost if the store doesn’t make any money off of selling items.
It is simply a matter of people taking the question at face value, rather than reinterpreting it the way that you have, that causes them to get what you consider to be the “wrong” answer. It doesn’t ask how much the store lost due to theft, it asks what its loss over the interactions we are given information for is.
If it will settle your stomach at all, reframe the question honestly, and ask how much was stolen, and I’m sure you will see far fewer come to an answer that you find objectionable.