Are they appropriating other people's land and building in their backyard? That would be called eminent domain.
They just want everyone to build what they want in their own backyard.
NIMBYs might more accurately be called NIYBYs.
They are telling communities that they have no part of how to manage themselves.
Rancho Palos Verdes should not be required to comply with the request of some random activist who probably has never even stepped foot in the town.
The use of "back yard" refers to the local area, not the literal extent of one's property. This usage is not unique to NIMBY and it's derivatives. YIMBY sentiment also clearly extends beyond developers themselves and simple libertine principles. Many people want development to occur around them, in their back-yard so to speak, because they prefer it occurs. The semantic change you're arguing for erases this concept just to sidestep the notion of local community. It's a needlessly aggravating approach when the simple answer is just that both NIMBY and YIMBY advocates can support their cause beyond their own area because they believe their cohort is right and deserves it.