Why is 50/60Hz interference is also called RFI interference?
Power lines get RF injected onto them

The power lines are a wonderful antenna, actually -- they get RF noises injected onto them by gadgets all the time via said gadgets' power cords (this is why we have conducted interference tests), and then proceed to happily radiate these noises out.

You are right that calling interference at the power-line frequency "radio frequency interference" isn't all that sensible, though -- most power-line coupling is closer to a near-field scenario than the far-field stuff we deal with when dealing with other sorts of RFI problems.  The catch-all term you're after is simply "electromagnetic interference", or EMI for short, as EM fields encompass both the near-field (either magnetically dominated or electrostatically dominated) and the far-field (both fields converge into a unified EM field) case.