This must be semi-regional because it's definitely not the case in the Minneapolis area (or the Midwest in general as far as I've seen). Even mom and pop restaurants will gladly take your credit card. If it exists at all, the minimum I've seen is $5, I've never seen a $20 minimum (not saying it doesn't exist, but it's definitely not "common"). Of course, the US is a huge place, so it definitely could be a regional thing.