I'm not sure whether it has always done it or not, but I do know that (with a 4 tuner Tablo) if you tune to a channel then switch to a different one that the previous channel will stay tuned. I'm guessing but I would think this is for a better surfing experience, so that now if you want to go back to the previous channel it is more immediate verses the 15-20 second wait.
I would expect that this is only the case as long as there are available tuners. If you are recording three shows than this would/should not happen.
In regard to your comment:
This sounds as if you may not be exiting Tablo properly. I don't think this should be happening, but I could be wrong.