I knew of the other rules but I've never heard of this one. I thought the whole point was to verify that the signature is legitimate. Otherwise, anybody could steal a credit card from my mailbox, sign it themselves and use it freely. If, however, they have to show photo ID, they wouldn't be able to.
As a cardholder, you are required to sign your card upon receipt. The merchant is then allowed to compare the signature on the back of the card against the credit card receipt.
If he/she feels suspicious, they have 2 options:
1) Decline the transaction
2) Call the 800# that is set up for that purpose.
As to someone stealing a card from your mailbox, think of it like this:
How often do you get a new card..maybe once..twice a year? Heck, for the sake of argument lets say 10 times a year! Thats a grand total of 10 opportunities for someone to steal your identity.
Now, how often do you use your credit card? By showing your drivers license (Which has your name, date of birth, address, even your SSN in some states) each time you make a purchase how many opportunites are you allowing? Several hundred a year? Several thousand?
Printing "See ID" on the back of your card and handing all of your information over to the minimum wage person working at Wal-Mart (or wherever you shop) each and every time you make a purchase is just begging for a problem.
EDIT:
And even if the do get your card during one of those 10 windows of opportunity how much are you liable for? Yep..thats right...zero(Ok..up to $50.00 in some circumstances.) Just report the card as never received/stolen and dispute the purchases.
The reason merchants ask for ID is NOT to protect you, its to protect themselves. If they let a fraudulent purchase through they know that the CC company will debit their merchant account(this is called a chargeback) and then they have to go through the hassle of filing the police report and attempt to recover the funds.
They would much rather put you at risk from one of their potentially dishonest employees (employee theft is the largest single cause of lost inventory or "shrinkage" in retail) than take that risk themselves.