Just added a Visa debit card to my Amazon account and it happily let me pay without entering a CVV code.
That's the 3 letter code on the back which prevents "just anybody" entering your credit card number that's on the front because if there's no CVV then anyone who gains access to your credit card can use it in theory.
Well I'm no expert but the banksters do some location, payment processor and other risk assessments on transactions as well etc. That's why stolen credit card numbers are sold with location data so the idiots doing the cashouts for the boss can shop in the same location and not raise flags.
So maybe it's not that dangerous but it seems a bit erm … imprudent that Amazon let me pay like that.
Yes it is normal.
As to why - not sure.
http://security.stackexchange.com/questions/21168/how-does-a…