You're right of course. It's probably because it's a lot simpler to understand, explain, and work with Newtonian physics, and there's most often no need to resort to G.R.Theory in order to obtain sufficiently accurate results. It's understandable, unless the aim is to teach G.R, but it would help the uninitiated if that were at least made clear at the outset.

"Time is what prevents everything from happening at once" - John Wheeler