Why do grandparents NEED alone time..?

So, I am so weirded out by this. My father and step mother have bought me and my SO multiple gift cards to go eat together. They even step up there amount on the card to try and entice us to have drinks and spend a night without our baby. I am not over bearing or anything. I let them come over as much as they want. Play with baby how ever they want. Other than kissing (Because they have cold sores). Like, why do you need to be alone with my baby. What makes it so much different if I'm not there...??? I also am mildly paranoid to leave my baby alone with them because my step mother stated she put rice in her nephew s milk despite his mother's wishes because "He was hungry." She also leaves and keeps formula at my house and her house just "incase" and always makes comments like.." You ready to formula feed." 😡 I don't feel comfortable leaving my baby with them. They think I have separation issues. But I will leave LO with dad no issue. I just don't see why they NEED alone time...Any insite?