That is the problem. If they give you whole house decoded then any device can record the content.
Also, I'm not sure you're aware of what the hardware would have to be to do what you request (I am an eletronics engineer and I do know what would be required). There would have to be tuner and modulator for each and every channel on the system in order to decode all channels simultaneously and then distribute them over the coax. Extremely cost prohibitive and the box would have to be monstrous in size (think over 300 RF tuners and modulators).
Are you certain that is the case? I had thought the STB was getting a digital feed of all the channels available on the subscription at the coax connection, much like any other cable system. Perhaps the cable box is doing more than acting as a tuner (ignoring peripheral functions needing the ip connection). I haven't tried it, but does the coax feed to the box supply anything when plugged directly into a tv having a digital tuner? I guess I could look around here and find out were I not lazy.
So I look at it this way ...
I add up the cost of the base TV service plus all of the STB's that I have in my house and get a monthly cost for having FiOS TV.
Then, I add up the cost of the base TV service plus the STB's (excluding digital adapters which aren't need presently on the cable company network -- although in my area that is changing shortly and every TV will require a digital adapter device) and get a monthly cost for havlng Cable TV.
Now I look at those two prices for the same service discounting the quality of the signal of one over the other or other items which might factor into the decision -- and I see which gives me the best value for the money.
You know what? For me, they're pretty close with at last check Verizon being slight cheaper month over month (and with a better quality signal and a few more channels). So -- Verizon presently gets my business for TV regardless of individual STB box cost.
I don't look at it as the cost of STB's but as the overall monthly cost to make all the TV's in my house work from each provider.