Everyone hosting ads would still get a portion of that particular ad's reward pool. Those offering the most to those hosting the ad, by spending more, aren't really edging out the competition because if thousands see the offer and select it, thousands have to share the benefits. If someone spent 10 for fees and didn't spend much more jockeying for position, and only one selected that ad, that one individual hosting the ad would get the highest share of the ad rewards, much like how curation rewards work. That ad might be a not for profit witness announcement, for example. Many already promote witnesses for free.

The benefit of clicking an ad goes to the host, not the clicker, in the form of a vote for the post hosting the ad. If it's a good cause, and someone clicks, the host still gets a vote, plus whatever portion of the ad rewards they'd be entitled to. If there isn't much in the ad reward pool, the host still gets the vote.

Someone could spend 1000 to get the best spots and offer the most to those hosting, but the market and share of the ad reward pool could mean someone who's spending far less is giving more since less people see it and select it to have displayed on their posts. That competition creates diversity, instead of everyone choosing the top ad with the promise of the most rewards. A balancing act.

Giving the option to host ads from all walks of life is just a courtesy. We have the 'decline rewards' option for posts. It's rarely used, but it's used. Some folks will see the benefit of hosting ads that offer less, and the click still functions as a vote that goes to the host.