Yes, once you hit the button, you're inside the board at that point (the board number you picked in the dropdown is sent through to the post form) so it knows which board to load permissions for.
-------------------
Why are the permissions not loaded? It's quite straightforward actually.
You have two boards, board 1 and board 2. The user can post attachments in board 1, but not in board 2. They can make posts in both boards.
The user goes to make a calendar event - at this point, they are not in EITHER board. The system checks, sees they can make topics in both boards, and shows this. But they're still not in either board, so neither board permission is loaded. How would it know which to load?
The post attachment part does not know how to say 'what about this board', it simply asks 'can the current user make an attachment' - since the user is not technically in either board at the time, no board level permissions have been loaded - because you can't load both sets at once in the current system. Which means the user won't have board level permissions for any board.
At the point of trying to make an event, which board's permissions could you even load? You don't know which permissions are to be loaded, and won't know until after the form is already made. You'd have to load every single board's permissions and juggle them around depending on what board the user selected which is really, really complex to do.