Unlikely that this would amount to an additional overhead - there will already be staff available, as evidenced by the fact that customers are able to pop around!
I don't know the ins and outs of a customer doing the factory tour but you would pay RRP for your chosen bat. This is higher than the trade price GM would sell the bats to retailers for, (theoretically) covering the additional time of the staff member conducting the tour. There may also be a charge for going on the tour, again to cover overheads.
If retailer came in to hand pick the same overheads would be incurred, as time is money. I don't know about you, but I get paid when I'm at work, so tasks I carry out cost the company money (time = money) so this needs to be recovered.
What is more cost effective, boxing up an order for 10 OLE bats, or showing someone round, letting them wave all the OlE bats round, test them with a mallet and chose the "best 10".
Quite frankly your statement about not amounting to additional overheads is ridiculous, there's no way it wouldn't cost GM money to run such a day (numerous times for each retailer...)