Your parking history is of no consequence - you've just been lucky for 20 years not to have been chased away.
The person (company) who owns the land can dictate their own parking policy - how they enforce it is another matter - worse of course are the services of a clamping firm so look out for clamping signs next time you park there.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix cars.