Let's do this socratically. Do you have home internet service? If so, does your ISP care how many computers are connected to the modem?
It depends... do the TOS state that you can only connect a certain number? VZW does.
Data thru a phone is much less than data thru other methods (PS3, XBOX360, laptop, desktop, internet enabled tvs for netflix). That is why it is unlimited to use on the phone. When you use your Phone to connect your other 8 devices, you are able to use much more data. That is why tethering is more. Previously, if you have unlimited on the phone @ 30 bucks, it was 30 bucks more to add tethering (capped). The reason was because it prevented people from trying to use their phone as their internet connection cheaper than getting an air card/ mifi @ 60 bucks a month.
Kinda like your employer paying for your gas in a company car and then you filling up all your family's other vehicles by siphoning it out of the company car every day. Yes, he is ONLY filling up your car.... but that is not how the system was designed.