Yes, it's entirely arbitrary that the Reagan Library debate rules include no-chancers Newt Gingrich, Rick Santorum, and Jon Huntsman, while excluding Gary Johnson, Buddy Roemer, and Thaddeus McCotter. And I wouldn't be shocked if it turns out that there was an effort to keep Johnson off the stage, given that a debate with both Johnson and Paul is a rather weird representation of the Republican Party on national TV. It's easy for the party to treat Ron Paul as a crazy uncle, but a lot harder to ignore two such candidates.
The framing of this is usually all wrong, however. What you have to remember about this is that fairness and equal treatment for all candidates is not, in fact, a party goal. Or at least, there's no reason for it to be a party goal.
Instead, the party should have other goals. It should try to facilitate cooperation and competition over the nomination with as little excess bitterness and fallout as possible. It should, if it wants to be democratic, try to ensure fairness and participation for all party actors. It also makes sense for the party collectively to try to put an appealing face forward during those portions of the contest that might get highly visible and ugly.
What all of this means is that the point isn't whether they're being unfair to Gary Johnson and the others by excluding them; it's whether they're being unfair to significant and important party factions. And for the most part, I don't think that's the case. Meanwhile, I do think it is very much in the GOP's interest to keep the total number of candidates to a workable number, and especially to keep the ratio of real candidates to joke candidates (or protest candidates, or whatever you want to call them -- the ones who have no realistic chance of winning) as high as they plausibly can.