R’ Moshe never issued a psak for “the velt” and never required “the velt” to follow his psak. If you feel there is an element of chutzpah to the psak permitting an eruv, go after the posek, not those who follow the posek. I have no idea who permits the eruv, but whoever they are they have a right (an obligation actually, according to R’ Moshe) to issue a psak, even if the conclusion differs from R’ Moshe.
Just curious. Did anyone ever tell Rabbeinu Tam, “who do you think you are, the zaideh Rashi paskened one way and 15 years later you have the chutzpah to argue and psaken differently?