In my experience it's been ignorance rather than laziness. I suppose you could argue that the root cause of the ignorance is laziness. If someone is writing a validator, they probably ought to check what constitutes valid input.
It's not a sign of catching on, but of using an old email validation pattern from the days of Matt's Script Archive (seemingly). It's the sign of a website that has used cargo-code programmers. You're probably just seeing it more because you're trying to use plus-addressing more often (confirmation bias), but it's been this way for years and years. It's actually better now than it used to be.
