I think there was a time when Mo refused to supply them with test balls as well, even though he was contributing author. (MoRich)
I would think the expense is negligible. My first thought was the effect of bad reviews. You give a ball an 8 or 9 in the category it is supposed to excel in, and now people aren't as interested. The companies that supply the balls are putting money into new products, and some slightly negative reviews could damage their sales.
In addition, the BTM reviews are purely based on the out of box condition. They explain in their reviews if the testers needed to add or remove surface/polish, but that isn't reflected in the grade. So if adding polish made the ball awesome on the short pattern, but leaving it out of box had a bad reaction, you only see the bad grade. As a supplier (i.e. Brunswick), I could see how some people would be put off by that bad grade without understanding the criteria upon which the grade was based.
I have no inside information, but this was my first thought. I should go back and look, but I think that a lot of the Brunswick stuff tends to get a tougher grade than some others ... deserved or not.