[whafro]

Well, he was banned for violating the Twitter rules, and those rules didn't exist at the time.

Getting a message saying something like "this message violates the rules today, so we're going to delete it" or "delete this message within a week or we'll have to ban your account" would seem more reasonable to me than immediate ex post facto enforcement.

[onion90]

I assumed that the comment had already violated rules at that time and simply was not recognized or discovered as such a violation earlier.