Most of the criticism of Load/Reload/St. Anger come from people heavily into metal. I mean, from what I'v seen and read and people I'v talked to, mostly online.
St Anger is fast, angry metal, so I don't see that as a problem. The problem is just that it's an overall weak album imo. And I don't mean that as empty bashing, as I do think it has a few good tracks, and some good ideas, but overall people just dislike it because it's not well written, or well performed, or well produced. Maybe it gets a little bit more hate than it deserves, but not by much.
It does play more of a part with Load/Reload, especially with metal fans who were into the band prior to that, and then had the curveball of some blues hard rock. That said, there are also plenty of people who dislike those albums because there are a lot of dud songs on there, and was essentially one album of writing padded out to a double album.
I do think a lot of songs on Load and Reload are among Metallica's best, and hugely underrated, so I do agree there are a lot of diehard thrash fans who hated those albums merely on principle (even here on DTF), but I'm also not going to discount that a lot of people gave those albums a fair chance and just didn't think they were good.
I listen to Metallica's non-thrash stuff like TBA, Load and Reload a lot more than I do the first 4 albums these days, so I'll always defend those albums, but everyone's got their own valid opinion.