You have to know about those terms changes, and companies aren't required to be real proactive about telling you in the U.S. They may simply send you an email saying "we've recently updated our terms and conditions" with a link to the current terms and conditions on the website, but no info on what was changed. So now you have to read through pages and pages of text looking for the new info, assuming you can even understand the legalese.
The acceptance of terms is always simply continuing to use the service, it's not a opt-in affair. If you don't read the terms changes, don't get the email, don't understand what's changed as long as you don't leave you've agreed to them.