Who says they aren't? Tejanos are top-notch Americans - I don't know why you are slandering them on zero evidence.
The law to build a fence has been passed. It is law. Build it. If they do not like it try and get an injunction.
I lived and worked all over the Texas border for many years and know many Tejanos as you call them. The ones I know are for the wall and restricting immigration.