I think it makes sense to clarify, to me it didn't seem like Gon intentionally "flipped a switch" and placed the nen restriction on himself. It simply looked like he became so emotionally unstable that by nature he powered up, not by active choice. Yeah he says "I don't care anymore" but there's a progression of him losing his sanity, which is why it isn't "bs that anyone can just do since they're going to die anyway". Maybe someone like Netero can do this by choice but I doubt anyone else has the ability to give up all their power for one shot.