They are reliable. Part of the UK Carphonewarehouse group. (The worlds largest independent telecommunications retailer)
http://www.phonehouse.com/. To bring it closer to home for you in the US they also own run the mobile bit of Bestbuy. (
http://www.bestbuymobile.com/)
To be honest I don't even know if they require a passport. I only once bought a SIM in a store in Spain and I just turned over the cash and got the SIM much the same as you do in the UK. If TPH (Thephonehouse) do ask for passport or ID it is so only so to see that your name is what you say it is. I don't think they will take down the number or details.
By the way, in Spain you even have to generally show photo ID if you want to pay for anything anywhere by credit card. (Anti-fraud). Spanish people have a national ID but for foreigners this generally means Passport, so you will have to get used to showing it a bit anyhow. I have got by with showing my drivers license in many places but loads of shops do not understand a non-Spanish driving license and do not accept it as proof of ID. It's a bit of a pain as I end up having to carry my passport everywhere!