The government does not impose such restrictions on the ip addresses and zip codes. What you are seeing is the site owners making a risk prevention decision to limit their materials to locations that are less likely to prosecute them for distributing their product. Every state has a series of obscenity laws that prohibit the manufacture, sale or distribution of materials that are considered "obscene". It is rare that prosecutions are attempted as these laws are problematic in the light of current First Amendment trends.
Here is a link to the relevant Texas authority...
You'll need to scroll down to Subchapter B : Obscenity.
This section will give you the specific codes that exist in Texas about the promotion, production, and possession of obscene materials. As you can see, the definitions are rather vague. More conservative states have definitions that are a bit more demanding that others, which is why some website promoters will make a risk analysis and decide not to allow users in certain jurisdictions.
Please reply if I can help further.
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