I'm a bit confused as to why you don't think this is a subsidy.
One problem is that your statement on having to pay the same monthly bill is not correct. I know that at least Verizon offers an off-contract plan, but the problem is that it's really not worth it as the discount is only $10. $650 > ((24 * $10) + $200)
Honestly, it's not that T-Mobile isn't providing a subsidized option, but it's rather that T-Mobile is providing a TRANSPARENT subsidized option. Other carriers don't provide this information and that allows them to continue to charge you the exorbitant rates regardless of whether you're on contract or not. Essentially, it's an incredibly dubious way of milking money out of your customers that don't know any better.
EDIT:
I like the transparency that T-Mobile is showing here, and I really hope this ripples through and causes other carriers to do it, or be forced by some governing agency to do it. Honestly, I highly doubt that we'll get the prior, and you'll probably only see it with the latter.
When my contract plan ended with Verizon, I was still paying the same exact monthly bills.
I had a contract on LG Voyager from December 2008 to December 2010.
I didn't leave Verizon until September 2011, and my monthly bills were exactly the same price from December 2010 to September 2011.