Thanks for the answer, that does explain the behavior I'm seeing through our testing. However, I do have some concerns that I wish you would pass along to whomever @ Telligent for consideration, both specifically with this issue and more generally with what I see as the "big picture" with how I think this came to be:
1. I've often run across technical issues such as this which indicate to me that Telligent was not designed with closed communities in mind. Another recent example was a bug that we saw in the pre-7.0 version which prevented the change password page from working (it keeps redirecting the process to the login page, preventing the password from being changed).
2. I do not see anywhere in your documentation for this endpoint (telligent.com/.../21799.create-a-user-6-1.aspx) that the welcome emails do not get generated. I've verified that CSModules DO get fired, which isn't consistent with the behavior you described above.
We're researching on our end now, but I'm almost certain that at some point, these welcome emails were being generated regardless of the method used to create the user. Our users are almost exclusively created through REST, and always have been, since we have thousands of users. I know this isn't a concern necessarily if you're an open community where users create their own accounts, but when you have a closed, customer-facing site such as ours, that's just not possible.