Which is why your server wants you to tip in cash, because then there's no official record of the tip.
P.S. to tipped employees, paying taxes sucks, but cheating yourself out of SS credits by not paying your taxes sucks more in the long term. Especially if you wind up with some disability in early adulthood that limits your work capabilities. You want your pay history with the social security administration to be as long and as high as it can be, so bite the bullet and pay your taxes on your tips too.
That theoretically makes sense, but I've never actually seen that. If I'm paying with card, it's almost always added to the total bill, so there's always a record.
Your server probably still prefers you to leave the line blank and tip in cash though.
But IMO it should be a measure of the service provided not an expectation pushed on the customer. A nice gesture given by the customer.
I remember a visit to the States where I stayed with friends, so got to see “Real America”, not just tourist traps. On my visit when we were just chilling out I remember my friend’s parents ran a Limo service and the discussion they had because one rider hadn’t tipped the driver, and who they were going to call them up and demand a tip. I can remember thinking that shit just wouldn’t fly back home and if I got a call demanding a tip after the fact I would just laugh my head off and put the phone down.
But it’s a different culture I suppose, one I still can’t get my head around.
EDIT: I see how it would be hard to change right now, IMO it would have to be gradual. Servers can easily make more than their wage just in tips, removing tips and paying the staff the same "wage" as if they got tips would have to put that increase of costs on to the products, which the avg customer will baulk at even if they are paying the same overall.