It seems like you've read of a trend, and are just guessing. It would be more productive to investigate the source of sites that cause this behaviour than aski

It seems like you've read of a trend, and are just guessing. It would be more productive to investigate the source of sites that cause this behaviour than asking random strangers whether a number of random (unidentified) websites are mining. We can't guess, we don't know. Nobody knows how widespread this is - but I suspect not very widespread at all.