Our problem will be the parent thinking I'm an overly strict gp. I will not allow certain things in my house and I don't care if your daddy's name is Obama. And there are a lot of parents out there who are raising little mini-me's who are being taught that it's only mom and dad rules and regulations that must be adhered to and worse.. that mom and dad trumps all.
I hope I never hear my son utter words like, yes it's your house but I told little Warren the he could throw the ball indoors. I think with grandparents it's a no win situation in many cases and it will come from infinite angles.
I think if someone is determined enough -- they will find anything to prevent a relationship that they don't want occurring.
When I say I'm the parent, it's my rules I don't mean that globally across the board. My rules are you go to someone elses house, you follow the laws of the land. DD knows and is aware that I parent differently than her father and stepmom but she also knows my #1 rule is to respect others. Their house, their rules. She also knows rule #1 applies to herself as well, if someone is not respecting her, she is allowed to do what she sees fit in order to correct it.
Now if I feel it is abusive, then I will be Mama Bear. Other than that, I think it's doing kids a disservice in not allowing them to participate in life and all the people in it. How do you expect them to keep a job with people from all walks of life?