Most campaign managers(top BTC campaign managers) have been in the system for a very long time, some are merit sources, spam busters, DT members, mods etc, their cv speaks for them, thus it'll be a stroll in the park for them, deciding if a post is worth a merit for whatever reason possible.
This is true for campaigns which pay in BTC, which makes sense; if a company are paying real money for their advertising, then they only want to hire the best campaign manager and the best posters. The problem is that BTC paying campaigns are the minority of all campaigns. The majority of campaigns are paid in newly created altcoins or tokens which the creators pre-mine, and so their advertising literally doesn't cost them anything. For these campaigns, they will hire any old manager who doesn't care about merit, and any old users with minimal or even zero earned merit to spam for them.
There have been several solutions suggested over the previous year, including removing or decaying all airdropped merit, removing signatures if no merit earned in the last x months, renting your signature for the cost of 1 merit per month, and so forth. I do think there is some merit (pun intended) to these suggestions; a lot of the spammers are either depending on airdropped merit or a single merit from months ago which was likely bought.