Zero Turn Mowers

My mother and I both bought MZ5225 zero turn mowers in Oct. 2012. Since new the steering arms loosen up to the point that you can hardly control the mowers. This has happened multiple times and have been tightened repeatedly and when we took them back to the dealer we bought them from and they said there was nothing else they could do for them. So I took them both to another dealer in town and when I originally showed them the arms, the manager agreed that something was not right. I picked them up today and all they did was tighten them again and said that it was normal wear and tear. The problem is the design of the control handle mount. They are round and mounted in a square mating piece and there is not enough surface area. When I went into the parts diagram, the original round part 539131521, has been replaced by a new part that is square (rectangle), 576452801.

My question is, since Husqvarna knows there is a problem with the steering, why will they not repair them under warranty?

I can buy the new parts and fix the mowers myself, but I should not have to since they are still under warranty. This is very poor customer service from Husqvarna.