It's a pretty tenuous argument, but the letter seem to be referring to Media Matters monetizing screenshots of highly political posts and in some cases ads being promoted next to those highly political posts.
While these articles have led some people to donate to Media Matters, the larger concern
seems to be other companies ceasing to purchase ad placement on twitter- which legally, none of those companies were ever required to purchase in the first place.
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While these articles have led some people to donate to Media Matters, the larger concern