Why would a company treat loyal customers like traders

I like playing games. I like buying games. I have a favourite internet shop where I buy almost all my games from.

We are a family of four (three of us live at one address), and they have a policy that means if I buy three copies of ONE game my account is then flagged as a Trader and all my account benefits and points are removed.

I will be wanting at least three copies of Guild Wars 2 once it is released, and I would like to give my money to my favourite internet shop, but with their current policy, I can’t do that.

As a business owner myself, I can not understand why on earth they would intentionally put a system in place that actively makes their customers live difficult, just to try and potentially stop someone from “reselling” a copy of a game, this really doesn’t seem like good business practice

I think companies should start putting a LOT more trust in their customers.

EDIT 1: I have made contact with the company, and they are looking into my request to have this policy amended.

EDIT 2: A 2nd account has been created at our home address, it does solve the problem, but it still feels more complicated than it needs be.