actually much of I-8 between Gila Bend and El Centro is within visual sight of the Mexico border, so it definitely falls within your 50 mile framework. But CBP actually operates within 90 or so miles of the border...otherwise they would not have an office in Casa Grande off of I-10.

If someone has a serious enough concern about what is and is not legal with regards to CBP operations and immigration matters, then I would highly recommend that one consults with an attorney for formal legal advice...preferably one specializing in immigration matters...in addition to reading the laws for oneself (something that I admit I have not done). Realistically, even my comments are mere speculation. Your mileage may vary.

We certainly are not going to change each others' minds today on such a hot topic. But if the original question is pertaining to what is and is not legal, then one really should ask a knowledgable lawyer instead.