It is a limitation of the trial version because they do not limit your tools like other companies. All "free trials" have some limitations in order to prevent people from using the free trial for a project but then not buying the software. There are some truly free (open source) video editors out there, but you get what you pay for - complex to learn, unstable/glitchy, etc. etc.
The trial is not meant to allow you to make a complete video, just to show you that the software is worth the money.