If you push this to the extreme, you'll need to see the source
code for the vendor's product so you can make sure they haven't
implemented some special "TCK mode". Obviously that's not
feasible.
Well, if we're talking about extremes, why not go the other way, and have no TCKs, and just trust each vendor at their word that they've been careful and done their own testing to ensure they implement the spec? But we're not talking about extremes here, so there's no point in bringing it up. Publishing an Arquillian adapter as open source is a low cost, high value thing that can be done, extremes are irrelevant when we're talking about such a cheap win. It also provides a very simple starting point for the frank discussions and compromises that you're saying should be done when necessary, since it means everyone can start from the same meaningful place, "we've run the TCK against product X and found Y". Plus most, if not all Arquillian adapters for Java EE containers out there are already open source, so in most cases, adding such a requirement primarily reinforces what is already done, it doesn't add anything new.