The registration page uses the label Email address (optional) to indicate that filling out the email field is not required. (When GrowthExperiments is installed, it changes the text to end with (recommended) instead.) In the mobile version, labels are hidden though and the placeholder is used to identify the fields, and that currently does not say anything about the field being optional. The email address is required on most web pages and all the other fields are required so the user will reasonably assume that we require the email address to be given.

On the other hand, the text Email is required to recover your account if you lose your password. (which would motivate the user to fill out the email address) also gets hidden on mobile.