Interest Groups not Shown for required lists

In the current iteration of the module, required lists are not presented to users at all, they are maintained behind the scenes via hook_user(), so there's no chance to select interest groups. Users can only choose interest groups, along with subscribe and unsubscribe and enter profile information, for opt-in and opt-out lists.

Is there a reason why required lists are handled separately? Would it be possible to change this in future versions?

Possible, but that would be a major architectural change and would be in a new version a bit down the road.

I second the requirement for this to be fixed in the D6 version. This is a major issue, since you are effectively forcing me to have two lists: one "required" list that is synchronized to the users on my system, and a completely separate second list that contains the special interest groups users sign-up for. That means my X-member account is made up of DUPLICATE email addresses just because your required list functionality doesn't properly handle special interest groups. In other words, I have to buy a Mailchimp account DOUBLE the size of the email addresses I actually have.

The mailchimp module was a core reason why I went with Mailchimp in the first place for my community site. I'm happy that you all have developed and maintain it, but this feature really need to be prioritized above "won't fix".

“And now our watch [for support of the 6.x version of the MailChimp module] has ended…” With the end of Drupal 6 support, I’m sad to say we too must turn the page.

Fret not! The 7.x-4.x and 8.x versions come highly recommended. Both are using Mailchimp’s new API 3.0 and are being actively maintained. “What is dead may never die, but rises again, harder and stronger!”