@Graham ******
It's not about abuse as such (why would Thai authorities care if you're not making any money anyway) but how labor regulations and "work" is defined. A secondary reason and probably a principal reason why they're fairly strict about ensuring farang don't volunteer in situations such as natural disasters is face.
Thais don't want to be seen needing help from farang (or any other foreigners). They want to do things themselves.
They do seem like they've got a hang on the flooding situation up in Chiang Rai and Nong Khai. Doesn't seem like they're even talking about farang volunteering there; in fact, they'd probably do everything to sabotage and threaten foreigners who help, especially those without work permits, which is nearly everyone since a work permit is tied to a particular job and you'd need a work permit specifically for volunteering to be allowed to help out.