".....I believe the real reason to exclude Libya, Somalia, Syria and Yemen is because these are countries from refugees are fleeing horrible situations and Trump does not want them in the U.S. There is no need to exclude Saudi Arabia. The Saudi's are happy enough at home and not an immigration problem....."
You are right on the money in your final paragraph. People traveling from war torn nations are simply more difficult to 'vet', that is to Identify, as to true identity, and run them through criminal history data files in their own nation, and our own files.
The idea is: why would you want to allow travel here from anyplace where you cannot trust what information you can get about that individual ahead of their arrival. In fact, ahead of the individual boarding to come here. These are high risk folks, until the U.S. can positively identify and certify that this individual is some one with a researchable history, who a Government we believe can vouch for, and answer questions about.
Does the individual have a VISA to enter the US? If not, they may not board.
Same as when any American seeks to fly somewhere other than here.
This is sound thinking, albeit imperfect, and not withstanding complications to otherwise welcome individuals which must be sorted, it remains sound thinking and worthy of being in place until and unless the US can verify that the person seeking entry is who they say, and is worthy of welcome here.
Jihadi trouble is one we need less of, not more of, and blocked are nations who are not friendly to the United States, for whatever reasons, with whom we have less than reliable communication, and who's information is not deemed to be reliable or in our interest to trust.
Saudi Arabia has a lot of odd folk living there, some very opposite views among them than the views of the United States, AND it seems we trust the information they provide about travelers headed our way, under their control.
Likely the same for Pakistan, if they are not on the list. Not people I want to trust, Saudi's or Pakistan, just my own hair on my arms standing up to think about who may come here from there, yet, that is not the question. The US believes information is valid, complete, and thus justifies good vetting on arrival, but no need to ban.
All too often, we seem to interpose LEGAL and illegal people, as to travelers or those found walking across our Southern border, and it is important to make the distinctions.
Illegal is 100% unwanted. Legal is ok, good to go.
Can't tell? Then unwanted until proven otherwise.