All the UI on this site talks about "Ask Question" and "Post Your Question". Those terms don't suit Meta Stack Overflow which is also used to accept bug reports, feature requests etc.
You are right, bug and feature-request and discussion are not questions, they are various other types of posts that pretend to be questions. There isn't a specific reason for this (that I am aware of) except that the meta sites (including MSO and the soon-to-be MSE) are special sites that were force fit into the Stack Exchange format.
Site regulars understand how meta works and how to use the site appropriately, but it does not make it easy for new users who are unfamiliar with our quirks.
Maybe there is a way to make it easier for new users to understand how to use the site.
Off-topic question on meta occur on a regular basis, most from users who get lost and ask programming questions on MSO (or on-topic question on the respective parent site), so there might be some logic behind modifying the UI slightly to help guide them.1
How about replacing the "Ask Question" button with 4 separate buttons on all meta sites. Each of the buttons can correspond to one of the required tags:
- Report Bug
- Propose Feature
- Get Support
- Initiate Meta Discussion (ok, this one is a little long, but maybe it can get tweaked a bit)
Clicking one of these buttons would open the new question page with the appropriate required tag already added. You don't have to change anything else about the page, just open the same new question page with the appropriate tag prefilled.
1 - I am not a graphic designer so I'm not going to even attempt at trying to show an example, but hopefully you get the idea