Is this a reasonable standard for shower attendance?

I'd say your "policy" is more than reasonable. A lot of times with these kinds of events, the hosts/honorees have a list of relations, and simply invite EVERYONE on it. Depending on the personalities involved, it's either out of a sense of obligation, or it's a gift-grab. But in any case, you're not obligated to attend, and they shouldn't be offended that you don't, as you hardly know them.