Don't know how difficult it may be to weatherseal push-pull zooms. Not interested in those.
What I am observing however, is that Canon is withholding wheathersealing in anything other than expensive Luxury products. In reality, wheathersealing of DSLRs and lenses is really simple and does not cause significant additional R&D and production costs. As demonstrated by Pentax over and over again. All it takes is a few O-rings in the right places:
That's what cheap kit-lenses look like at Pentax these days:

http://www.photoscala.de/Artikel/Pentax-K-50-Wetterfest-und-robust [and sorry, got it in German only at the moment]
I think we've seen example after example over the last few years that suggest Canon's philosophy is to give consumers as little as they can get away with and still move product. Exhibit A: the T5i; Exhibit B: the 50D, complete with video capabilities that were disabled prior to release.
Now, before anyone says anything, I'm not saying this isn't their right. It is something to be aware of before jumping on the Canon bandwagon. Pentax, for example, offers more for your dollar but if you want to use a high end tilt-shift lens or an elaborate Speedlite system Pentax may not be the best choice.