In IAH I think that when you leave the baggage carousel area through the Customs gate you are leaving the secure area; I am pretty sure you can go from there to the exit doors to ground transportation without going through any other door that you cannot re enter. In Hobby you are clearly out of the secure area on the way to your connecting flight because you pass right by the ticketing counters.
But of course I agree that there is no apparent (to me, anyway) reason to route continuing travelers through an unsecure area.
Just to follow up, I returned to the U.S. last night and the doors on both ends of the re-check area - after customs and before ground transport - are literally covered with NO RE-ENTRY signs and warnings, plus after exiting the last doors after re-check you are in an enclosed corridor for another 50-100 feet with lots of arrows and more big warning signs at the end to not enter.