Disclaimer: See my sibling comments for some my general thoughts on the problems with banning trademark ads.
But for your specific example - I get where you're coming from, but I'm skeptical that the ad market is even that functional.
Firstly, if I google "leatherman", every sponsored result for Leatherman brand multitools anyway. (And no amount of refreshes or re-searches gives me anything other than Leathermans.)
Secondarily, I'm not convinced that the set of advertisers (not counting Leatherman itself) that will advertise for "leatherman" are actually on average a better products for the consumer. (e.g. as opposed to lower-quality, higher-priced knockoffs.)
These are both fair points (generally, the consumer market is pretty dysfunctional and not behaving at all like economists would like it to), but the comment I was replying to ("It ought to be illegal to buy ads against trademarks") seems both too heavy-handed and unlikely to actually do any good.