Like Pro Ag said, that is just how I ordered it online.
You don't have to actually switch to T-Mobile. Cricket runs on the AT&T network (which is a GSM network, not like Sprint or Verizon which are CDMA). When you order the phone online one of the steps is to choose your carrier. In the past you could also pick the "carrier free" option, but for some reason they removed that option. The only carrier choices that they offer to choose from are AT&T, Sprint, Verizon, and T-Mobile. Since T-Mobile and AT&T (and cricket) all run on the GSM network, any phone that works on one will work on the other. However, if you pick AT&T as your carrier option they make you verify your phone number to ensure that you actually have AT&T service. If you pick T-Mobile, there is no number confirmation.
So you can keep your service with cricket, order a "T-Mobile" iPhone from the apple website, and pop in your cricket sim card when you get it.
Works like a charm.