I think it comes to analogies, with open source you have a public park you are free to use. With source available it is public park you are free to look at behind a fence... So not actually public park. Still a fine thing to exist.
As user as well. Difference between I can use this for free and I have to pay to use this. Even if I can see parts inside is significant.
It might not be real principle, but at least it is real difference.
Wouldn't it be more along the lines of "source available" being a public park that you're free to access but can't monetize by e.g. selling tickets to - while "open source" would let you do whatever you want with the park.
Ironically I think the analogy explains why many people find "source available" to align with their moral compass more than "open source" necessarily does.
Source available is a park where there is some set of activities you are forbidden from doing in the park, but you're never sure exactly which. One day you realize your driveway goes directly into the park, and if you transit to your office through the park you get sued. The park vendor is also trying to get you to buy a home inside the park.
As user as well. Difference between I can use this for free and I have to pay to use this. Even if I can see parts inside is significant.
It might not be real principle, but at least it is real difference.