"With Canon, you sometimes get the impression that if you're not using a full frame camera, you are a second class citizen."
Its all about money. Its a food chain. The whole point of this, that if you want to take photography seriously, you cant get stuck at APS-C. What do you think, why theres no weather sealed EF-S lens? For the same reason. If you want to go pro, you have to go fullframe.
Then why is the 7D, which is the next best thing to the 1D's weather sealed?
but then, the 7D is your sports and wildlife camera, typically with long focal range lenses (ie 70-200 2.8/4 is, 70-300L, and all the big white primes). bird/animal hunting and field sports especially don't get interrupted by a bit of rain, so you need a weather sealed body for that.
i've got a 7D, and i love my 15-85 for wide landscapes. but i also know in the back of my head, that if i was going to be more serious about landscapes and sweeping panoramas, then i'd be saving for a 5d2 + 16-35.ii. Panoramas tend to (ok, not always) look better when it's not raining. what else is the 5d2 good for? lowlight, ie weddings, indoors (start trails? yeah, when it's not raining...) and other things that don't need a weather sealed body.
or just look at the simple answer: the 5d2 is a year or two older than the 7d, and it takes a while to get new tech into cheaper things. yes, they had weather sealing already way-back-when the 1n and 3 film cameras were around, but digital has more buttons and plugs to seal up. adding weather sealing to the 5d2 at the time of release would have pushed back the release date and price up.
also, canon obviously knew the direction they were heading with the 550/60/600 all coming out with the same sensor, you have to make the most expensive model worth the extra money, so they sealed it up knowing the 60d would be coming out later and had to be justifiably cheaper.
and yes, by that logic the 5d3 should definitely be sealed, but you never know...