Law enforcement has wide discretion in deciding who to cite for potential wrong doing and who to let pass by unremarked. Prosecutors and regulatory bodies have similar authority.
This shouldn't be remarkable or unfamiliar to anyone. We see it happen all the time, both to ourselves and to other people. Some people get ignored while committing a moving violation, others would get pulled over and get a warning, while others get a traffic citation. The "shady biker bar" gets a threat for running free bar poker, the Mexican food place a few blocks down the road running the same game one day earlier in the week never hears a word.
These poker rooms are, at best, in a grey area of the law. I personally think they are black and white unlawful. If no one complains and the room isn't causing a problem, then they get ignored. The police and courts have plenty of other, more serious, things to spend their time upon. However, there are locations in Texas where poker isn't considered harmless. Maybe it is as
@dkersey suggests and the legal cardrooms in other states feel harmed - enough so to put pressure on the local LEOs to enforce the law. Maybe it is a local religious group who feels Jesus is offended by gambling. Maybe a local cop had someone in his/her family get into gambling addictions and feels compelled to act. Maybe the Friday night crowds grew large enough to upset the neighbors.
The bottom line is every card room is left unmolested at the discretion of local and state law enforcement. The last six weeks or so, said discretion ran out in the Dallas area. Next month it could be Houston, next year San Antonio while the card rooms in Austin are never subject to the threat of legal action. Yes, they are being discriminated against. So what? That happens all the time, not just in Texas but in every state of the union. Equal treatment under the law is, at best, an aspirational goal. More likely, "equal treatment under the law" is the story peddled to children, people of privilege and politicians running for office.
You play in the games, you take your chances -=- DrStrange