For ease of math, let's say you see one ad for every ten tweets, this effectively limits a single users ad impressions to 80 day. That is not something advertisers would have expected when they dropped dollars onto the platform. As an advertiser, you also can't be assured going forward that Musk isnt going to randomly implement some other major change that effects your business.
I'm guessing the rational here is fighting against scrapers harvesting tweets for AI. Whether this is effective on that front, and whether worsening the user experience is a worthwhile tradeoff, I don't know. But it's smart business to at least give people, users and advertisers a heads up first. It sounds like Musk implemented this change Saturday morning and didn't announce it until he tweeted about it hours later.