Agreed. It's very odd indeed - it's obviously intentional since they could easily have stopped signing these versions along with all the other older versions that they accidentally started signing, but makes little sense.
The only other case of an old iOS version being permanently signed is 4.1 on the 3GS and I remember reading somewhere that they did that in order to allow OS X 10.4 users to restore their phones through iTunes, but this obviously isn't the case with the 6.1.3 signing for 4S and iPad 2.
Only apple knows why downgrades are allowed on these particular devices but I believe it's a good thing. I personally use 9.3 on my iPad 2 - it's been used every day since 2011 and still runs great. But those who want to downgrade should be allowed to, especially on older devices that are pretty much obsolete no matter which firmware they run. Apple really doesn't have anything to lose by letting those of us who are still happy with old hardware use it on whatever firmware we please.