I agree about the SGA tie-ins, although the main thing there is that they just. Weren't very good. And of course there are definitely fannish venues that are nearly as restrictive.

But most of the early Star Wars tie-ins made all sorts of massive status quo changes and had very few limits on what could be written (no gay stuff or porn, mostly). The early Star Trek ones notoriously often *were* fanfic that the editors just bought, no continuity concerns, with variable amounts of editing, and they didn't do a very good job keeping out the gay stuff, if they tried. The Doctor Who tie-ins basically went wherever they wanted and are their own thing now, and the IDW comics for Buffy and Transformers were so good (and transformative) they're basically a lot of folks' main canon.