Naming functions with reserved words

I noticed in the temporal project they used reserved words for their
functions (union, intersect, etc)

But when I try to create a function like that I get an error and I have
to quote it both when creating the function and when calling it. The
only difference I can see is they their functions are written in C and
mine are in SQL. But that doesn't make sense why you could do it in one
language and not in another.

What am I missing? And if it can be done, is there a strong preference
in the community about whether it should be the reserved word or follow
PostGIS's tack and prepend something to all of the functions like
ts_union and ts_intersect?

Scott