Its their carpark, they can do as they please with it. If I owned a large carpark I'd be pretty pissed if people were using it to spend money at another store...
Is it? I can't see why it is outrageous, when you consider the cost of hiring the clamping guy, the lost opportunity cost of not having a parking space available for customers wanting to shop at the supermarket etc. Again, its their carpark and they should be able to do as they please.
I don't blame him, I know a few tow truck drivers and having an attitude is pretty much part of the job. Its not like most people caught say "fair call, you got me, I was breaking the rules", if you had to deal with people whinging and making up petty excuses every day you'd get sick of it too.
Contradict much?
In short, if you don't like it then don't break the rules. I can't stand people who moan about parking tickets, speeding tickets, etc etc. At the end of the day you were doing something you shouldn't have been and you got caught, so deal with it and suffer the consequences...