It seems to actually work in some circumstances. One Russian company was
founded by a chemist who had research papers on their website. He was
convinced that the small MHD effect caused by moving water in a magnetic
field would cause mineral crystallites to line up and bond together
(flocculate) as blobs of gel, where normally they stack up as stone-like
encrustations.
However, sometimes the magnets do nothing, and nobody knows why. If any
company hides the fact that these magnets are very unreliable, then that's
a group of scammers and ripoff artists. Also, if the product is far more
expensive than a handfull of magnets, then it's a total ripoff (I've seen
these things sold for $500, and they're just a couple of blocks of ferrite
magnets!)