By writing "SEE ID," a malicious cashier can now get your credit card and ID-- enough info to impersonate you in some scenarios, make online transactions, deliver things to your address and steal them, etc. Yes, you have to show it in some transactions (purchasing alcohol, for instance), but those are rare. If you're asked for it for any reason at all (ex-- "to prevent fraud") that's a red flag, and a violation of the vendor-card agreement. It's not like the "SEE ID" thing would protect you from fraud when so few cashiers actually look at it anyway.