If you move people's threads instead of deleting them, a subpopulation of posters will simply post EVERYTHING they write in the default forum, waiting to have it moved by a mod later.
There is no default board; clicking Create a Discussion either defaults to the currently viewed board or requires the user to select one from a dropdown. As for posting everything to a single board expecting moderators to move it, this just doesn't happen in a well-maintained environment. Users will occasionally make mistakes, but as long as repeat offenses are treated correctly (if you expect mods to do your dirty work you'll be muted or banned) it's not a particularly significant issue.
Do many people do this? No, but it's enough to make your life a living hell.
As I briefly mentioned above, good moderation policy would harshly punish people abusing the moving system. If you have a pattern of intentionally posting in the wrong place or not caring about the location, it's pretty clear you don't deserve to be part of the community.
A huge number of people post things in Boards Feedback (the default)
Again, there isn't a default. A user has to either select Boards Feedback from a dropdown or navigate to the specific Boards Feedback board in order to post here.
By taking a deletion stance, they're teaching people after one go that, yes, you do need to post in the correct place. If they took a movement stance here, Boards Feedback would be getting a ridiculous amount of traffic that ultimately all had to be moved.
This is flawed logic. Deletion might teach people that they are posting in the wrong place, but it doesn't do anything to show where they should be posting. I understand this isn't the eventual goal, but currently there is usually one place (maybe two) where a thread belongs.
I could make the same argument by saying that throwing students out of my classroom is a great teaching tool as opposed to sitting down with them and explaining why what they are doing isn't ideal. Sure, throwing them out gets the point across, but it's not likely to make them like me or like my class.
If they took a movement stance here, Boards Feedback would be getting a ridiculous amount of traffic that ultimately all had to be moved.
Something people seem to be forgetting, users post about deletions all the time. The fact they are so unclear and so sudden leads people to be confused and frustrated with the platform, which can simultaneously cause people to leave the platform and give moderators/Riot more work by potentially needing to address these concerns.
Also, Boards Feedback already gets lots of posts that need to be deleted. Why is the immediate assumption here that moving will significantly raise that number, assuming repeat offenders are properly controlled?