Is it fair, humn,
I believe that is a question that would be answered differently by every person on the forum.
If you feel that it is unfair then it is. I have worked with many, many snakes from colubrids, pythons, boas and venomous and most do quite well in rubbermaids and that is how I start with all my neonates into adulthood. After they become breeding size they all get there own custom cage of apropriate size.
People sometimes take the cage size way out of proportion and I love to see cages that are big enough to walk into. After you see cages like that a six foot cage for a 5 foot snake seems unfair.
So the bottom line is your snake will do well in a rubbermaid of apropriate size, the larger cages are just for our own conscience.
A bad year with reptiles is better than a good year without em.