The ban on Saltspray Rainmaker is because of the combination of crane shenanigans and the fact that there's only 1 choke point to enter the northern area. It can become very easy for a team to maintain control of that point and prevent the opponent from entering at all, especially a team as strong as Squid Box. Even then, there are still seconds wasted in order to wait for the crane to move to an appropriate position in which players can jump onto it.
Kelp Dome's issue is mitigated by the fact that it's deep inside the enemy's spawn point, which is supposed to be hard for you to enter anyways. That kind of cheesing can still be allowed, where once you have the lead, you take the rainmaker into your base and camp hard. But at least your opponents are granted several different ways to try and enter your base.
Basically, Saltspray potentially gives a team 2 "base camps" with hard choke points, which can be hard for the any opposing team to deal with.
Concerning OOB exploits, it is impossible to enforce player action rules. For HHB, the reason that we didn't ban it was because Danny simply didn't know it could happen in that fashion. For that same reason we can't enforce people not getting onto the crane.
We're not banning the maps out right. Saltspray and HHB will still be in rotations for Zones and Tower. Also, we're changing up the algorithm to ensure RM's don't happen as often. It should be noted that RM, SZ, and TC appeared in similar numbers, just that their distribution was off.