This setting is enabled by default. Admins have the ability to exclude some forms fields from being pre-populated from an email link through this setting that has been added for each profile field.
To set the field to NOT pre-populate, the admin needs to uncheck the option. Administrators may want to set the email address field to NOT pre-populate, especially if this field is used as the username for the site.
When the Allow data population is set to "off" for a profile field on one form, it will be set to "off" across the site. For example, if Event1 and Event2 both use the email address field on the form, and the admin disables the pre-population setting on Event1 it will also be disabled on Event2.