This is documented behavior, but we see lots of questions on it. A good mechanism for preventing users from accidently exporting reports to a format that you don’t want (for example, you might know that the report doesn’t render quite right in a particular format) is to mark the extension as “invisible” in the Report Server config file.
This MSDN documentation explains how to configure a rendering extension:
The important one here is the Visible attribute:
A value of false indicates that the rendering extension should not be visible in user interfaces. If the attribute is not included, the default value is true.
By setting this attribute to false for a given extension, it will hide the extension from being visible in the Report Viewer control as well as in the delivery configuration pages.