It's the videos of course
As many posters have already pointed out, there is a lot of flash that gets downloaded by "normal" users on full computers. And much of the really "heavy" flash are entertainment downloads - like TV shows, movie clips, etc.
Mobile companies, on the other hand, have pay-for services that provide many of the same feeds you can get for free on a full browser computer. They really want to defend that source of revenue, as this is one of the few competitive revenue streams they have. Blocking Flash and limiting tethering are the only options to preventing erosion to this revenue stream.
Personally I'd pay the extra tariff for full tethering ability: it's just too nice to have when sitting in boring meetings with my tablet PC. On the other hand, my Pre browser is pretty darn good, arguably equal to or just behind the iPhone for general WEB surfing. Especially if you have the WiFi enabled and are in a fast hot spot.
Alas, it *IS* a phone after all, and I can live without tethering for now. I am confident, however, that this feature will be allowed in the future. This seems to be the hot topic at the Sprint stores from customers looking to buy the Pre, and Palm is quite aware that this is also at the top of the list of the new owners. With Apple/AT&T still restricting tether on iPhones, and Sprint offering unlimited tether with most of their other devices, this issues still has plenty of time to play out.
BTW, it seems that the OTHER hot topic at Palm on the Pre is the problems associated with pairing to several built-in auto hands-free systems (Lexus, BMW, Accura, Honda have been mentioned). This is probably higher on the list than tether right now.
Also, just in passing, the Pre is missing an OBEX profile, severely limiting the utility of Bluetooth for sync and file exchange. It does (allegedly) have a PAN profile...working on how to tap that myself...