People have a history in the immigration system, so whether tourism is an excuse or not, it won’t change how officers target individuals. You can’t pretend to be a tourist if you already have a history of excessive exemptions; be honest and that's it.
As mentioned previously, holding a visa doesn’t protect you from being interrogated. This is how it works in Thailand and certainly in many other countries. Since the DTV is one of the easiest and cheapest visas available in Thailand, it’s not surprising to see a growing number of people being questioned when trying to enter. I’ve had very bad experiences in the US and Singapore in the past, even though I was completely legal; that’s just the job of immigration, and people need to understand it. And yes, dealing with a grumpy agent isn’t pleasant, but most of the time, when crossing a border anywhere in the world, I see the same behavior.