Why do people feed my kid!!

Okay am I overreacting.. I literately lose my shit when people feed my 17 month old food without asking me and off the fork/spoon they have eaten off of. Why do people think this is okay!? We were at a party and my sister in-laws fiancé fed my daughter cake, right after she had her own cake. She didn’t need anymore cake and especially off his fork! Like I don’t even let my own mother do that, let alone someone who I’m not related to. What do you think??