It sounds like you're confusing two things — or at the very least the two are tangled in your question:

1. Ubuntu membership is open to all. Anybody can apply for it and their work here on Ask Ubuntu can contribute towards that membership application (speaking as an Ubuntu Member who got through almost wholly on his AU work).

We don't "offer" it because no sub-partition of the support community does. We can't create members directly. Just like good IRC users and good UbuntuForums members, we go through the standard membership process.

2. We don't display membership slang next to users because the only thing that gets displayed next to the username is the post date and their reputation data. That's internal to Stack Exchange and that won't change.

Ubuntu members are more than free to advertise their membership on their profile.

It sounds like you're confusing two things — or at the very least the two are tangled in your question:

1. Ubuntu membership is open to all. Anybody can apply for it and their work here on Ask Ubuntu can contribute towards that membership application (speaking as an Ubuntu Member who got through almost wholly on his AU work).

We don't offer membership because we do not have a community council just for us. IRC and UF both have their own councils with which to create members. AU users step through the traditional regional membership boards.

To pre-empt why don't we have a council?, I think there are three main reasons:

  - Moderatorship and Council member election work completely differently. Moderatordom is permanent (until you die or give it up) while community councils need to be regularly re-elected, with open nominations.

    In short, there could easily be a conflict between the two groups.

  - It's more work its members. CCs are supposed to resolve conflicts, they need to hold regular meetings and in the case of sub-boards (as we would be), we would need to report to a higher board once a month. Assuming this would be moderator lead (at least to begin with), we'd just be wrapping what we already do in a ton more red tape.

  - Do we even need it? 

   - For the purpose of becoming a member, you can go through the standard channels. 
   - For resolving a dispute you can either come to meta, or if it's deeply personal with one moderator you can go to the Ubuntu Community Council or Stack Exchange. We moderators are already accountable to both groups.
   - For anything else (planning, whatnot) you can just use meta to discuss it.

2. We don't display membership slang next to users because the only thing that gets displayed next to the username is the post date and their reputation data. That's internal to Stack Exchange and that won't change.

Ubuntu members are more than free to advertise their membership on their profile.