Gonna just reiterate - The problem isnt mailing to your alts in of itself. The problem is how if you were to allow a direct trade to alts, the rational pragmatic thing the entire player base to do is create more alts to get more storage space. That is a purely rational decision people would make, as you are saying. But in doing that, people will also rationally say "Why should I pay $2 a month more per extra retainer when I can simply pay 1 for 14 extra retainers." So most people do not purchase retainers for extra storage space.
That is a direct revenue loss for SE and the Devs. That loss translates into impacting the game - Either by cutting development in places or compensating the loss into bigger sub fees. This is the 'downstream' effects of just one change in the system. Ironically, You are actually agreeing with me and my point about alt banks and providing SE a financial incentive not to allow mailing between alts.
As for punishing players, the reason why they probably dont is because its not cost effective and hard to prove. So they compromise. Knowing that there are loopholes in the design, theyve put up walls that make it difficult or inefficient to use alts as storage for most players. Theyre likely operating on the concept of simply saying "Yeah, we know you can do this method and turn alts into storage, but most players would rather just spend a few dollars more and not deal with the hassle."
Beyond that, how do you expect them to 'stop' a trade between alts? Lets say you want to transfer items between alts and you have a friend act as the intermediary. What is a realistic way for SE to stop that? Create a special invisible flag that denotes whom the item originally came from? How long does that flag stick around? Does it stick around if the item gets placed on the MB. When its purchased does the flag go away? Or how bout the item is traded through a few people. Not to mention now you have a new and completely separate background tagging system that the servers have to manage and track.
Lets think this in reverse though. Forget the monetization issue - why dont they just straight up allow you to mail/trade to alts? Afterall, it would be a QoL improvement. What reason would they exactly not allow it - especially if the "It stops RMTers" is bunk and RMTers are getting around the system anyways without issue? I mean, we're not gonna assume that the devs are sitting there being like "Yeah screw these players who want alts! They suck! hahaha so fun to make things bad for them," right?