This behaviour of the acceptance checkbox might confuse some form submitters. Hence, it's highly recommended that you set it up in a way that will let the users know why the message won't fly.
So, How Do I Make It Work Like Other Validation?
==> Sign in to your WordPress dashboard
==> Go to "Contacts"
==> Click "edit" next to the contact form you want to modify
==> Add this line below, to the Additional Settings section:
==> Click "Save".
Try using your form without checking the acceptance box. It should display something similar to the screenshot below:
I hope this helps.
Recommended: How I Use Free Contact Form 7 Plugin For WordPress Blogs