[§5Aki]

Yes, being in spectator would make it easier to cheat with a buddy, etc. However, I'd say the main reason the rule is there so people don't just sit in it to be afk. Then it's taking up a spot someone else could be playing. If you know you're going to be afk for just a second just stop playing and step away. However, if you're going to be gone for a long period of time (aka longer than a round or 2) then just disconnect from the server rather than sit in spectator.
This rule also applies for people who will join the server and sit in unassigned for up to a whole map. Which is why there's a rule for that too.