No, don't worry, a USB port is fine and makes sense since it saves having to use one of those USB-PS/2 adaptors that aren't supplied with mice any more. It's just a matter of being more clear with the description since there's a very real risk of unwary people being misled. There are other products out there that only support PS/2 mice with a USB socket, and they're always very clear about the protocol required so it hasn't been an issue. For example
I'm sure it's a good product and a good price and I wish you luck with it, and I hope this hasn't put you off further development for the platform.