“Let’s say you live by Grandma, so for convenience, we're going to put you on Grandma's bank account. Guess what just happened?” Picciurro says. “If you're put as a joint [owner] on a bank account with somebody and you have the right to take the money out at any time, essentially Grandma is giving you a gift.” This applies to joint accounts when the other owner is not your spouse.
A lot of people have joint bank accounts with elderly parents or relatives - for the sake of convenience and security. How the heck would the IRS enforce a stupid rule like this.
By having the banks report it to them.