You may be correct; I've seen a number of articles specifically claiming Amazon and Expedia joined the lawsuit (which has a clear and well accepted meaning), but on review there are also a lot claiming that the relation and there involvement is somewhat different, and I can't immediately find any definitive documentation of their joining as plaintiffs (they clearly weren't initial plaintiffs). Their involvement by any of the reports may still be of a kind to make it inappropriate for them to be amici, and they clearly support Washington (and Minnesota) substantively in the case, at any rate.