Why are the STL files for the Ultrascope telescope at 45 degree angle?
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.