I agree. It's not like with a metal key where a "lost" key is a major security issue in addition to the cost of the key; a lost card key is completely useless to break in and steal from the next guest. One of my favorite hotels charges $25 for a lost key, but it's a real metal key; totally fair IMO. It probably costs them more than that to re-key or replace the lock.

The card key itself costs less than 50c even in very small lots. Put a pizza ad on it and the hotel gets the keys for free.

I can use way more than 50c of water and energy with a long, hot shower and the A/C on full blast for a few days--should they charge me extra for that as well? How?

I've lost more mag-stripe keys than I can count--they're easy to lose! If a hotel charged me to replace it or for forgetting to return it, you can bet it'd be the last time I stayed there. It's not like a bathrobe, towel, TV etc. that you have to steal.