I don't think I've ever been asked anything about what I'm planning to do at Thai immigration, and I've gone through dozens of times at various airports and land borders. The only thing they care about is that you don't exceed the validity, and obviously you can't actually work (as in, get paid locally) while in Thailand.

Also, rally, I've never heard of people in suits getting randomly stopped in Bangkok -- where did you hear about this? If you have a valid, non-expired tourist visa, there's nothing the police could possibly do.