Calling a woman sweetheart is extremely offensive, because she isn't your sweetheart.
It's like calling a woman your f*** buddy, when she isn't. Even if she was, it would be offensive if you said that to others about her, and especially not in front of her. But to a random woman you're not in a relationship with, it's even more offensive, somewhat like calling her a prostitute, and saying you own her cause she's yours.
This should be obvious.
I don't know why this would be difficult for anyone to see.