Libraries "weed" books all the time, and most of them are pulped. Which is about like burning them, only the carbon doesn't end up in the atmosphere. So this is probably not the best comparison.
This is a totally wrong analogy. Books were burnt for religios/political reasons. Storify is giving up because of lack of financial instreams for the owners.
Tons of valuable books and records have been burnt because someone thought they weren't valuable or didn't have the money to house them. It's true, it's more likely they were just thrown on a trash heap and left to rot, but trash incineration is a thing.
Burning books because they aren't "valuable" is dangerous ground. Safer to store everything.