They can only hold you for a search if they have a good reason, and your refusal is not a good reason, legally speaking. Besides, even if you are caught with a gun, the law says that THEY have to prove you're not travelling. Unless you tell them where you're going and where you're coming from (and you don't have to) then it seems to me it would be pretty hard for them to prove you weren't travelling.
I'm not sure what the Houston DA is up to--he may just be trying to scare people into doing what he thinks they should. If it turns out that he really is acting contrary to law or instructing the police department to act contrary to law then he would be in trouble.