Technically you can keep it once you claim it for customs, there's nothing stopping you from doing it. The only issue would be that CO's computers would show it as a checked bag and expect it to be on the aircraft. Bags are counted/scanned as they are loaded anyway, and they're not going to wait for luggage, but if something were to happen and the baggage counts and manifest were audited there'd be a descrepancy. CO's system would probably show it as a mishandled bag as it would have been scanned into IAH but not out, despite being tagged to SEA.

They will not short-check without a stopover though. If you had a day or two in IAH then they'd only be checked to IAH, but as you're on one routing the airline will not short check them just to IAH. You can ask, but security protocol says they shouldn't.

I'd imagine if you leave the tags on it, you'll get some questions from airport personnel. Let's not forget that you can check things like firearms in checked luggage, so they really don't like you turning your checked luggage into carryon luggage mid-trip.