I see that. I couldn't find any justification for that choice either. Any reason I can think of (security, legacy, optimization) would imply the powers that be wouldn't want CSP allowed as a meta tag either. My guess is it's just another bad choice.
I've had iframes completely disabled in uBlock Origin for years now and the only time I've found it "breaking" ("fixing" from my perspective) things is when sites do something like third-party run sweepstakes. I think we'd be better off just breaking iframes now for the sake of our children, like we did with Flash et al. Start off with "click to activate iframe" with harsh warnings, then completely remove support shortly after. There are better solutions for sites that are using it for legit reasons.