Events - Registration Confirmation and Receipt (on-line) is not sending to anonymous users
Ah - ok - this is what's happening:


if the event is FREE -> then no receipt is issued via CiviCRM workflow message templates 
if the event is FREE &amp;&amp; if participants make a donation -> then the participants get an email message from the CiviCRM workflow message template - contribution online


So your workaround is a good one - a simple Email message that is triggered via CiviRules (add Participant to a certain event) - or you could even do a simple Email message from Webform -> Emails.