Is it unethical to distribute 3D models of Lego bricks?
Almost certainly no, but with some caveats:

The LeoCAD parts library is based on the LDraw parts library, which has a Legal Agreement here.

The vast majority (and I would assume the entire subset used by LeoCAD) are covered under the Creative Commons Attribution License 2.0:

You are free:


to Share — to copy, distribute and transmit the work
to Remix — to adapt the work
to make commercial use of the work


Under the following conditions:


Attribution — You must attribute the work in the manner specified by the author or licensor (but not in any way that suggests that they endorse you or your use of the works).


However there are parts in the library that are not covered by the LDraw Contributor Agreement (CA), and these are covered by the more restrictive NonCA agreement. You are expressly forbidden from sharing, or making derivative works of these parts (creating 2d renderings of the elements isn't considered a derivative work, but converting the parts to a different format is).

All part files should have a license comment in their header stating either:

0 !LICENSE Redistributable under CCAL version 2.0 : see CAreadme.txt


or

0 !LICENCE Not redistributable : see NonCAreadme.txt


You should check those parts you wish to adapt, and proceed accordingly.