I'm curious why food producers didn't do this on their own, since no one likes "sell by" anyway? Or was there an existing regulation that got changed?

In any case I think the new way is better.

They don't want grocery stores to carry product that's too old because customers will try it and decide they don't like it, not realizing that it's old. And a grocery store worker won't know when to get rid of it unless someone tells them.

And from a grocery store's point of view, product that's on the shelf too long isn't selling. It's taking up space that could be used for something else.

I'm pretty sure food producers are the ones putting the "sell by" labels on stuff, with the hope that people will needlessly throw away food and buy more because of that label.

Presumably food producers benefit if people throw things out earlier, as they will buy more ?