The OP is actually quite accurate in using the word "punished".
To people who said "because the game ain't designed that way" you miss the point. While the structure of the game make it content more 'accessible' on one character I don't think it's equivalent to "encouraging people to play on one char", much less "discouraging playing alt". To be frank, if it wasn't for the ability to make more character, I would not use a standard sub at all. The default sub give you 8 character slots at a higher price, so saying the system isn't "designed" for it doesn't really fly. Also to be able to play different class is just one of the many reasons people have alt. Glamour, race, weekly allowance ...etc... which are all possible here, so again, "not designed for it" doesn't fly.
But the strongest argument here is you don't even have to look for specific "alt support" and argue it's not meant to be. There are things that are lacking not because the game is not capable, but because the dev specifically went out of their way to prohibit you to do so. The most commonly requested and arguably most severe hindrance to having alt in this game is the lack of ability to transfer item between alt. For those of you who either are new here or weren't present when it was possible, it's actually possible to mail stuffs between your alt. The mailing restriction is due to you can only mail people on your friend-list, and since 2.0 there have been instance here and there where players found ways to friend their alt, and thus allowing them mailing stuffs back and forth. And each and every single of those time, SE patch out the method in short order.
As for the reason why you can't just friend your own alt? Last time I check I think they used " 'cause it encourage boting" as the excuse. Well, I let you be your own judge on how flimsy that excuse is, considering bot has no problem transferring stuff with their exclusive bot-FCs anyway.
So yes, given the fact that it's a high-demand yet extremely basic QoL feature for alt as well as already being already possible in the game (since 2.0), the fact the dev went out of their way from keepign you doing so indeed is a punishment. Again, that maybe a strong word, but it is also accurate.