Marriott screws guests all the time but I don't understand how they did anything wrong here.

You see a rate you're happy with. You're incentivized to prepay because you're given a discount. The terms are disclosed. You agreed to them knowing full well that hotel prices change based on demand. Again, you're happy with the rate.

If you prepay a non-refundable rate and the prices go up before check in, would you accept the hotel charging more?

Sometimes you win prepaying, sometimes you lose. If you don't like the rules. don't book that type of rate.