@Scott *******
You are looking for logic where there is none to be had.
IF you're going to be comin' here on an O-A visa it will be "employment prohibited" AND no matter what you think (or what you found in engrish googling around) volunteering DOES fall under work in the eyez of the thaiz. Now whether that makes sense to you or not is totally irrelevant. It only has to make sense to the thaiz.
Believe me I'm not tryin' to piss on your parade, I'm telling you you're certainly NOT the first foreigner who wanted to volunteer here BUT due to the way the rules that define what constitutes work AND the rules that govern the issuance of a "work permit" <-(needed to volunteer for free) they couldn't do it.
Good Luck,