Okidoki. I indeed already checked the ThaiNIC website. It's because of these ridiculous rules people just go for a .com other whatever:
"in.th An individual as well as a company/organization can register domain names under .in.th, and must send a copy of ID card (if registered by individual) or a copy of Por Por 20 (Certificate of Value Added Tax Registration), Tor Kor 0401 (Certificate of Company/Partnership Registration) and/or other related documents (if registered by company/organization) to ThaiNIC™ at fax no. 0-2939-6499. Please note that a foreigner, who wants to register a .in.th domain name, must have a legal work permit and reside in Thailand."
Ofcourse the customer can deliver it, but a 15 minutes task turns out to be a weeks hassle this way!
I'll ask them if I just cannot register 'DontStepInTheTurd.com' and have their business run from there......