It's just a convenience thing, since the carrier-specific nano SIM comes preinstalled. If you swap the SIM yourself, then it becomes a non-issue, mostly. I've heard Verizon is a jerk when it comes to activating devices that are not certified by them, so getting a Verizon SIM could be challenging for some.
Ya, I couldn't wait for T-Mobile to get their act together to allow us to order it in the store/website the day the Retina models started showing up in the stores, so I got a bunch of free nano-SIMs from T-Mobile when they had that Veteran's Day sale. Then, I used that
Sea Turtle website to locate any iPad mini Retina with LTE in the color and capacity I wanted, went to the store, ordered it using their Macs, hung out for an hour until their Personal Pickup was ready for me, and I popped in the T-Mobile SIM before I even powered it up. BAM. Done.
I have the Verizon nano-SIM in a pouch, ready to be used if I want to add it to my family plan (but, with the way they're fumbling with their network reliability, I may just toss it).
None of the iPads are preconfigured, branded, or locked. It's all the same iPad model with the different nano-SIM cards inside. The moment you power it up, it will figure out what carrier it is and provision itself as needed. The only difference is last year's white/black iPad mini without Retina that, while still unlocked, don't have all the LTE bands. I popped in a T-Mobile nano-SIM in my 2012 iPad mini and it says "T-Mobile 4G" and not "T-Mobile LTE" as it shows on my 2013 iPad mini with Retina.
Since Apple prides themselves on the "perfect customer experience right out of the box," they chose to go this method of 32 different SKUs for their cellular models - four capacities, two colors, four carriers. If you're smart or impatient, just head to your carrier of choice and buy or get a nano-SIM card, then do what everyone else is doing and get whatever cellular model you can get your hands on. You won't ever have to worry about activation issues or calling customer service, as the activation is completely done on the iPad.