I agree with the change to default to site settings time zone id for new users. Whether timezone is required at registration is determined by the mojoProfile.config file or a custom version of that file. I have also changed it to make it required by default. These changes are now in the repository, you should revert your changes and get the latest code.

We recently changed to a new timezone system from our previous system which was only an offset from GMT. The old system did not automatically adjust for Daylight Savings Time which varies by time zone, the new system does automatically adjust for daylight savings but it doesn't work in Mono, so we still support the old system under Mono. 

When we added this new system we had to account for existing users where the setting would be an empty string, in this case times are adjusted to the site default time zone until the user updates their profile which would set their timezone. So this was also happening with new users, it would be unspecified if it was not yet set by updating the profile. But going forward we will use the site default and/or the choice made by the user during registration if it is required for registration.

The profileview page shows the display name of the user's time zone correctly during Daylight Savings but it is different when using the dropdown list for editing purposes we don't adjust names for items in the list we just show the id name like "Eastern Standard Time" instead of Eastern Daylight Time, it is the same time zone and the official name and the id is "Eastern Standard Time", but during daylight savings we refer to it as Eastern Daylight Time.

Best,

Joe