The law isn't super clear here, but nobody has put up a serious legal challenge against this practice since Rosetta Stone sued Google over it back in 2012. Google was awarded summary judgement against Rosetta Stone for its practice of selling bids for searches for Rosetta Stone's trademarks:

https://en.wikipedia.org/wiki/Rosetta_Stone_Ltd._v._Google,_....

There have been earlier legal challenges to this practice as well, but none has resulted in a clear opinion that it is unlawful.