Steps to reproduce the issue

1. Use the Joomla 3.9.16,
2. Turn on the option Register users on your site,
3. Go to Extensions -> Plugins -> User - Profile, select a value for Terms of Service -> Required, but DO NOT select the article with the conditions (below),
4. Go to the front-end and try to register, but DO NOT check the Terms of Service checkbox.

Expected result

Error because I did NOT check the Terms of Service checkbox.

Actual result

I registered on the site without any problems, but did NOT check the Terms of Service item. Please note that the hint tells us the following:

System information (as much as possible)

Joomla 3.9.16
PHP 7.3.2

Additional comments

If you select an article with conditions in the plugin settings, then everything works correctly - the field is required and registration is not possible.

You need to either remove part of the tooltip from the language constant:

PLG_USER_PROFILE_FIELD_TOS_DESC_SITE="Please read the Terms of Service. You will not be able to register if you do not agree with them."

Or, check for an article with conditions and, possibly, do not display the field even if the value is selected, which is mandatory.

In short, the logic is strange and needs some refinement :) Thanks.