I think the main question is whether or not these phones are special 'unlocked' or possibly SIM-free stock, or simply the same AT&T/VZW/Sprint/T-Mo phones you would get if you simply either used a carrier upgrade or applied EIP or bought out right like in previous years...
If they are - then they will probably have their carrier's respective SIM Cards and/or ESN IMEI identifiers and thus will require intervention to 'tie' the new SIM/ESN/IMEI identifier to the account (probably post paid if they are requiring SSN, address, zip code for all 4 carriers) in the system. That is where typically they setup activation fees to auto trigger if this occurs (of if the attendant helping adds it).
This could possibly even trigger a contract agreement or extension, who knows the carrier's backend reaction - the dangerous part to me is that no one at Apple knows for sure and I don't think they have a lot of visibility as to what happens on the carrier side once the POS purchase API pings your respective carriers allow 'activation' and purchase on the upgrade plan.
This lax, uncharacteristic lack of information from Apple is not a mistake - I just don't think they know for sure period and will literally let the chips fall where they may as to each carriers, specific "terms and conditions" when it comes to this process. Very dangerous and I advise forum mates to be vigilant on their bills.
This is the point in which any sort of 'apple-endorsed' iPhone upgrade program can differentiate itself by not requiring this process but the language of the small print sounds very familiar and I would not be surprised if the above is the case, unfortunately.
Nevertheless I plan I applying and trying my luck with a t-mobile phone, mainly because I am tired of this dance and I'd like the ability to throw the 6s onto Apple's wood tables and request a new 7.