Square does support PIN entry: you just enter the PIN into the touchscreen of the mobile device. Square developed a way of securing the PIN that doesn't require dedicated hardware. (It's now a PCI security standard: "Software-based PIN Entry on COTS".)
Yes, there may be some markets where Square doesn't support PINs. I took your comment to mean that Square doesn't support PINs at all, and that it's because of a physical security requirement. I couldn't resist responding to that, since neither of those has been true for a while.
I'm not going to enter my PIN into someones mobile unless I have a way of verifying the app they are using is really the app it says it is. I can't see how that could possibly work.
