Not true anymore. You can get either Samsung Galaxy SII or Galaxy SIII and even HTC EVO 4G LTE from Ting a Sprint MVNO and at very good rates and interesting plans. I will go for it, except that the speeds in Sprint and the related MVNO are not even mildly exciting.
The problem I see with Ting is that they are expensive. For example, looking at my T-Mobile $30 plan, an equivalent plan on Ting would cost over $100. The Virgin Mobile $35 plan looks like it would be over $60 with Ting -- and much more than $100 if you take advantage of the unlimited 4G.
Last, while they do have two high end phones in the Galaxy SII and SIII, you are still limited to the phones they carry. So, you are limited to three Samsung phones, the Optimus Elite (for $40 more than VM) and one Kyocera. Not to mention, as you pointed out, being on the Sprint Network -- particularly with the SIII not currently getting 4G in most areas of the US. With the SII you can get "4G", Wimax, in most large cities but can't get LTE as it is rolled out.