T'Girl: I wasn't done, but okay, I was unlikely to shut up anyway.:P
It's strange, I agree, but that arbitrary point is, if not canonically
established, then overwhelmingly accepted among fandom...Personally, I think it makes sense as a "If you don't have the resources to place Observation Teams everywhere indefinitely, use this as the somewhat arbitrary marker for when you can pull out Observation Teams and say hello". Especially given the Prime Directive, it rapidly becomes apparent that you have to use something
as the boundary point, and FTL travel (whether it be by warp engine, artificial singularity, or whatever) seems like as good a point as any to go from total quarantine to "sending over the welcome wagon". (One presumes that anybody who has FTL travel either has or will get FTL comms and/or FTL sensors very soon...And especially later on in tech development, it's probably a race to be ready to contact the newbies before they: A. Discover ET by going bump; B. Discover ET by Something Appearing On Their Sensors; C. Find FTL comms and act like a toddler with a CB radio.)
Part of the problem with social, cultural, and intellectual measures that aren't technological is that they're very subjective and, this is no small part of it, subject to backsliding. (Not to mention very hard to observe from a distance.) I don't recall cases of widespread technological backsliding (besides the Fall of the Roman Empire), which is a more objective measure (either you have it or you don't) in any case, and much easier to observe from a distance. Remember that most Federation observation efforts are going to be conducted not from ships within the system (where anybody with a telescope can spot you eventually) or teams on planet, but with sensors outside the system.
I'm not totally sure where to take this writeup, that said. I have a post more or less done in Notepad about "After First Contact", or should I junk that and go directly to cases where First Contact is made with someone who's had FTL travel for a while?