Sigh, you really do protest too much, Andrew.
The reason this is bad is perfectly simple: it establishes the principle of a two-tier system. Once you get that in the door, it's very hard for anyone to later complain when they change the 'details' of the system.
So of course they introduce it with 'details' that seem nice and unthreatening: instead of charging more for things outside the scheme, we're charging less for things inside the scheme! Isn't that nice? Charging less is always good, right? We're not even charging partners to sign up! See, it's free! Who could possibly complain about free?
Well, the problem is the pricing numbers are just numbers. Once they've got the trojan horse in the gate, they can twiddle the numbers any damn time they like. Two years down the road the scheme will cover a lot more large, rich companies, but suddenly you'll be paying twice as much for data outside the 'scheme'. But who cares, right? All your tweetfaces are inside the scheme! A bit later they'll make some mumbly noises about overheads and start charging providers to join the scheme, but who cares, right? We can still see our tweetfaces for free!
Then just wait until they need a revenue injection and start re-introducing charging for data within the scheme...and oh look, we've got the differential pricing that was supposed to not be allowed in the first place.
There's an argument of course that there's nothing really wrong with allowing this sort of tomfoolery in a reasonable simulacrum of a free market. AOL and Compuserve got competed out of existence, we didn't need laws for that. But cellular network markets are rarely particularly good examples of ideal capitalism.