What they are targeting, without kickbacks or other backroom deals, is a service by which users on mobile devices get a downgraded version of a particular service which most likely accounts for 80% or more of their traffic. As long as any provider can opt in without T-Mobile charging them anything more than a reasonable one-time administrative fee I don't see any problem.
I, as a T-Mobile customer, have the ability to opt out which is critical. If I lived somewhere where I could only get slow DSL or no wired Internet at all I might want to be able to stream via the mobile network onto a large screen. Then I pay for the privilege, and that is okay, but as of right now I have Binge On enabled because I don't need high resolution video on my 6" cell phone screen.
Also, consider the number of people who pull up Youtube videos just to listen to music...