Yeah I agree that one seems silly on the surface but for their specific situation I understand why: services like Gmail allow using a + to create faux-labels. So for example foo@gmail, foo+bar@gmail, and foo+baz@gmail all get delivered to the same account. For change.org that’s a problem because it allows a single email account to fill out the form many times.
Ideally, they would simply truncate everything after and including those symbols but it’s possible other services have different rules (maybe yahoo let’s you prepend faux-tags instead of appending them, or something like that) so simply blocking their use altogether could be the more robust solution