Users with a "Chatter Only" SF license do not auto populate in email address searches

In a recent case with a customer, we learned that when using email services such as "Email Assist", you are unable to search for users in your org that have a "Chatter Only" Salesforce license.

For example, if I'm sending an email to an internal user with a standard Salesforce License and I type in "John Doe", "jdoe@gmail.com" will appear in the dropdown list of selectable recipients.

Now if I need to send the same email to a user with a "Chatter Only" license and type in "John Doe", no dropdown appears with selectable options.

The end result is that I have to go back into SF and find the "Chatter Only" user's email address and then manually type it in.

I'm told this is expected behavior but I'm wondering if there's a specific reason this was designed this way and if there's any plan to allow the search dropdowns to see "Chatter Only" users?

Thanks!