"We think others should be sued too" is quite a different argument from "we don't know why we're being sued". In fact it strongly suggests the opposite.
They don't know why they're being sued because everyone else is doing it and not getting sued. Makes sense from that point of view. Coinbase technically should only be sued if the others doing were sued too, but they're not... Compound this with the fact that no explanation was given to them why they're being sued - when it is very much necessary in light of their competitors not getting sued and you can start understanding their confusion.
