As it stands, the form does what it says, people just don't read carefully enough.

The other way, if someone says, "Ah heck, I didn't really want a new topic" and unchecks the box, it doesn't do what it says that it will do.

The JS solution, ugly as JS is, gives the streamlined experience, but also always does what it says that it will do. Which has value to me.

Cheers,
Ben

PS Second thought. You could lose the checkbox and have the dropdown default to blank. If someone sets the dropdown for where to create a new post, then they wanted a new post in that forum (possibly the current one). If that is blank, then they didn't.