<p>I began printing the parts for the Ultrascope DIY telescope designed by the Open Space Agency.  See <a href="http://www.openspaceagency.com/ultrascope" rel="noreferrer">http://www.openspaceagency.com/ultrascope</a>.</p>

<p>All of the STL files for the 3D printable parts are canted 45 degrees.  Brackets, tubes, everything I have seen so far.  Is there a reason for this?  I printed one part last night and simply rotated the part so it would lay flat because I didn't want to deal with supports.  I am relatively new to 3D printing -- Am I missing something I should know?  Is this a thing?</p>

The orientation of the part in the STL file depends on the Software that creates the file. I had a software that would export the parts standing upright instead of laying flat. Depending on the CAD software it can be beneficial for the creator of the model to create in in a different orientation as the one you want to use for printing. Also not all CAD Engineers know (or care) about the best orientation for printing a part.
So my guess is that this is an issue of file export/ STL file creation.
It is totally normal to rotate the parts into a position that is best for printing.