If we can stop people from holding down reload button we might keep the uniqueness of spas and still be fair to all.
I don't see "having to reload manually when the gun is not empty" as a very unique aspect, every other gun works like this too. What the other guns don't have is the ability to load one round at a time and therefore being able to approach the combat in different ways. This of course wouldn't be changed in any way.
I understand that runners would do it to further increase their speed, but running is not running if it's made easy is it?
Running is one of the reasons for the suggestion but definitely not the biggest. The Spas isn't the only weapon the runners use and honestly speaking how you're reloading the gun doesn't make much of a difference if you're running, meaning that reloading is the easier part. Like with any other weapon, you reload whenever it's possible, the only differences being that you can reload one shell at a time and other weapons aren't used when there are no enemies around. Also everyone knows the basics of boosting with the Spas and you can't make it "difficult" no matter what.
Top reasons I'm suggesting the whole thing are conveniency and pure logic. You don't have to worry about spamming reload to make sure the gun is reloading regardless of the situation, and you're going to be reloading it anyway so why not make it automatic. Why don't we just remove the automatic reloading of empty guns too?
3rd reason: Reloading the Spas when the key is set to mwheel_down is super awkward. 4th: TF2 has this too. Optional, but anyone who knows better is surely using it.