I certainly use "no beta" tags/notes as a data point when deciding whether to check something out too, along with the summary, author's note, tags etc.

It's just my guess that the tone of these notes intentionally highlighting the lack of a beta (rather than for example just not mentioning it either way) might be a reaction to having been scolded for not using betas in the past (whether personally or as a feeling of a general fandom mood that translated into feeling scolded) and pushing back against that. It's my impression that there's always a bit of a seesaw in what take on fanwriting is the most vocal in a fandom corner (a bit depending on BNFs and cliques popularity at a time).