You are using a service to access another service. In fact, you are using a service (internet isp)to access another service (xbl/psn) which allows you to access to your game service.
So yeah, really. I think reading the ToS will explain it fully for you.
[spoiler]You might be interested to know that at any time, Bungie or Activision can block, restrict, or cancel your access to their service. It's all in the ToS. [/spoiler]
English
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Yo dawg, I heard you like services. So we made you buy a service to buy a service so you can use your service while you use your service.
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That just scum bag business. Blocking our access to games was an overreach in developer rights due to a lack of laws with games. I'm still waiting on a pricing board to get on with esrb to stop the bullshit pricing of non content. This industry is falling apart because of it.