IANAL, but this is a reminder that something being in a TOS/EULA doesn't necessarily mean it's enforceable. There are contract laws and consumer protection laws that make certain conditions illegal and not enforceable. Sometimes companies include these things out of ignorance, and sometimes deliberately under the assumption that consumers don't know what their rights are. If you are suddenly surprised by terms you think are unfair and may cause you harm, then ask a lawyer.

Also, as some people of pointed out, this isn't actually part of Prezi's TOS, so I'm very doubtful that this would stand up in arbitration/court.