Yes it's perfectly legal.  

There is a Wireless Service Agreement that you accepted that specifically states tethering is against the Terms of Service unless you have a plan that specifically allows tethering.  Your paying a per device connection.  Cellular data is a premium due to limited bandwidth and is regulated differently.  So if your jailbreaking your phone and using the feature without paying for it, your violating ATTs terms and basically stealing the service.

Your TV service is not always as you describe.  A lot of places the cable channels are scrambled and you have to pay for each box for each TV to get all the channels.  Also with your home phone the service is limited.  While you can hook up multiple handsets they cannot each use different services on the same line.  You cannot call different numbers/people from each phone if you only have one line (thats an extra premium for each additional line of access).