As far as your second claim ("all other buy/rentals are iTunes store based"), I'm surprised that you have full access to and complete knowledge of the Apple TV source code since I doubt that you work at Apple.

There could
I don't need access to source code, dude. It's plain to see that certain movies change from rental only to buy only and back to rental AND buy and/or buy SD only to HD, etc. if you keep track of any movies what-so-ever. I didn't get software updates here for those changes to happen. The software hasn't been updated for a long time. I can back-date my ATV to Version 2.0 (in firmware) and those same rental/buy changes still appear here. It's the store that determines what you can buy/rent/preview, etc., not the software. It only cares about the format.
be a very simple technical reason why the ATV1 software can't support TV rentals, such as knowledge that the show must time out 48 hours after you begin viewing even if the Apple TV is completely disconnected from the internet and the iTunes Store.
Yeah, the 1st gen ATV doesn't know ANYTHING about timing out rentals....
I'm not claiming that these ARE the reasons, just that they MAY be the reason. Also, the original Apple TV contracts with the TV studios may not include the rights for rentals and Apple and/or the TV studios may not have wanted to renegotiate those terms for the installed base of ATV1 users.
I don't see a difference between a license for the two models. The content should be the same (M4V). Even IF new content for the new system is 720@30fps, they still obviously have the 720@24 available because they're selling it. I doubt it would be that difficult to rent the stuff that is already converted and available for sale. They probably need little more than to be flagged rent-able like the movies.
In any case, if there is real money to be made in TV rentals then I'm fairly confident that Apple will update the ATV1 to support them. However, you've got to remember that several of the participating TV studios have already suggested that iTunes TV rentals are at present a test or trial program -- there is no guarantee that they will even continue.
The whole Apple TV experience is a trial, it seems. Jobs has no interest in advertising them. He has no interest in listening to user feedback. He has no interest in supporting old hardware, whether it's iPhones or Macbook Pros. Apple has purposely kept new features from working on older models even when they are 100% capable of using them (from video on iPhone 1.0 to H264 decoding on 2008 era Macbook Pros). You simply cannot count on Apple to support their products for more than a year these days. Part of the reason is that Apple WANTS to force new hardware sales since that's where they get most of their money.
However, the problem here is not the price of a new Apple TV (they're cheap) but rather that my receiver does not support HDMI switching and the new Apple TV does not provide component output. Thus, It's not just $99 for a new Apple TV to get whatever new features I might prefer, but I'd have to buy a new receiver as well (probably $400 minimum for the same brand base model, considerably more for higher quality models) or use an outboard switch. So whereas it might have cost Apple $2 more to include component output on the new model, it will cost me $500+ to switch over and thus the primary reason I'd rather see rentals available on the old model seeing as most are already available to BUY in HD on the same model. Offering a rental version of the same video would not entail much on Apple's part (regardless of whether you believe my take on store versus software). Their attitude seems to be, too bad. We don't support software updates for the old model anymore, as if it's our fault that they kept using Tiger all these years and waited so long to update the hardware, which still offers very little in the way of improvements over the old model so far other than price (and removes a lot of storage).
I'm not saying I'd rent a LOT of TV shows, but there has been the odd occasion where I miss a 1st run episode or cannot get the HD version of the show on my cable system where it would come in handy. I guess Bit Torrent would be more handy than Apple for that situation given the lack of support. After all, that seems to be the way the studios want it by not offering flexibility. They want you to buy BD and/or DVD full seasons of TV shows, not simply rent an episode you missed. I don't call that reasonable, personally.