Once at DL, our over-3YO made it through in his stroller without us scanning his ticket. Then we couldn't get FPs for him without going back to the front of the park and getting his ticket scanned. You could run into the same problem in WDW if your magic band didn't get scanned. So they are doing some a favor by double-checking that the tickets are scanned. It's possible they aren't really looking for bad actors here, just trying to ensure everyone who has a ticket is getting it scanned. Given that I've never seen or heard of anyone being challenged on the age, I'm going to assume the best!