Hidden within Donald Trump's executive order on immigration is this little gem, section 9:
Sec. 9. Visa Validity Reciprocity. The Secretary of State shall review all nonimmigrant visa reciprocity agreements to ensure that they are, with respect to each visa classification, truly reciprocal insofar as practicable with respect to validity period and fees, as required by sections 221(c) and 281 of the INA, 8 U.S.C. 1201(c) and 1351, and other treatment. If a country does not treat United States nationals seeking nonimmigrant visas in a reciprocal manner, the Secretary of State shall adjust the visa validity period, fee schedule, or other treatment to match the treatment of United States nationals by the foreign country, to the extent practicable.
"Nonimmigrant visas" are visas for business, education, and tourism. The ringer here is that a few nations, most notably Saudi Arabia, prohibit tourist visits entirely: this website states that Saudi Arabia allows foreign visitors only for sponsored business travel, religious purposes, and family visits. Conspicuously absent are visits for education and tourism.
If this portion of President Trump's order is upheld, and it might be, then he has directed the Secretary of State to treat Saudi visitors the same way that Saudi Arabia treats American visitors: that is, to keep them out of the country. He's told the Secretary of State to exclude from the United States all Saudi nationals who are coming for education, or for non-specific business purposes, or simply to visit the United States -- even though Saudi Arabia isn't one of the nations on his list. He's actually moved to exclude from the United States citizens of one of the nations that produced the 9/11 terrorists, even if he hasn't realized it yet.