Using a MIFI is no different then using your cell phone. It has a phone number just like your cell. so how are they gonna tell the difference between a cell phone and the jetpack??? Many folks use their cell phones as the MiFi device too....
There actually is. Cellular frequencies and WiFi frequencies are very different and do not conflict with each other. When you turn on your phones Hot Spot (or connect to McDonald's WiFi) you are no longer using cellular frequencies but rather one of the 11 WiFi channels (on 2.4 Ghz I believe) available. There just is a misconception that 2 devices on the same channel will create a conflict which is not true. It "can" be true, but the reality is that many devices can operate simultaneously on the same or side by side channel without issue. Only channels 1, 6 and 11 do not interfere with each other and yet if you click on the wireless networks available to your computer right now I would bet that more than 3 networks are listed.
If you like to see what's going on around you and actively participate in trying to keep yourself on a clear (or as clear as possible) network, you can download InSSIDer and it will give you the channel and signal strength of the surrounding networks. If the park is using 1 and 11 you can set yours to 6 just to be perceived as a good neighbor.
What you see looks like this:
http://www.metageek.net/products/inssider/Currently I am showing 18 nearby networks.