T-Mobile never said that you could finance a new iPad with them, without being a customer. They addressed this specifically in their press conference. A recording of the press conference is here:
http://newsroom.t-mobile.com/phoenix.zhtml?c=251624&p=irol-newsArticle&ID=1867397
It's simple. Everyone gets 200MB of extra data for free, regardless of whether you bought your iPad from T-Mobile. Period.
If you want T-Mobile to offer you 0% financing for 24 months, you need to be a customer. You can have a cell phone plan, and pay nothing for tablet data, and still get 200MB.
If you don't have a cell phone plan, you can pay $20 per month for 700MB of high-speed data (500MB plus 200MB), and unlimited low-speed data.
Why would T-Mobile offer 0% 24-month financing to non-customers? They never said they would, specifically said "you'll need to be a customer," and it would make zero business sense for them to offer it.