Holding a spot for a family member who ran to get a drink or a snack I think is absolutely fine.
What I might have a problem with is one person holding a big spot for like 10 people and those people are riding rides etc....and arrive at the last minute. But I still wouldn't do anything about it though since I'd be there nice and early anyway to get our own spots.
We also get a curb spot very early for our family of 4 and then one of us, or sometimes my DH and girls, run into the bakery or casey's corner for snacks and I hold the spots with our backpack, ponchos and stroller. I don't think I'm doing anything wrong. We all wait for most of the time except for the running for snacks.