What he's saying is that the device could be attacked - obviously not by design, it's designed to not allow the private key to be read by issuing commands to the Trezor. But depending on the chip they've chosen, physical possession of the Trezor by an attacker would allow him to run other types of attacks (power analysis, etc.) to extract the private keys from the memory.

I don't really expect any merchant to allow you to just randomly walk in and plug a device (_any_ device) into an USB port on their computer. Especially one that implements the HID protocol (presents itself as a keyboard). See http://hakshop.myshopify.com/products/usb-rubber-ducky for an example of what I mean.

As far as I understand, the Trezor is meant to keep your private keys secure in case your computer is infected with malware. It's not something you would use at a merchant.