We've been hauling a road motorcycle or two for about 6 years and have never had to pay a second fee for either of the bikes. Of course I've never stopped and asked if I had to, either. I always operate under the notion that it's easier to ask for forgiveness than permission.
If they came around and wanted me to pay a fee for a second, or third, vehicle then I'd pay, and I would leave as quickly as possible. I hate being gouged. Making you pay for a bike that you hauled in, and takes up very little parking space is ridiculous. Reasonable fees are one thing, but arbitrary fees are something else entirely.