Still, it is not much of an incentive. The vast majority of people are either going to leave the site due to performance issues or dislike of it. Of those who stay, the majority of those are going to upgrade their clubs at some point in time anyway. Putter Pal or not, they do so for better game performance. I honestly don't think a free Putter Pal is really going to influence that purchase for the vast majority of players.
Your original suggestion indicates they should give it permanently. That means after the first purchase you've received your Putter Pal and have no incentive to buy yet another set of clubs. A better incentive might be to give a normal Putter Pal or a sleeve of premium golf balls with club set purchase. If the player likes those free products they will be inclined to purchase it after that free sample runs out.
Regarding players with disabilities, I'm all for those players working with WGT to get things at low or no cost that enable them to play this game. I believe that should be addressed on an individual basis by the company.