It has everything to do with the fact that these are "proof coordination networks" and without open source you have no reasonable way to know the structure of the proofs, or what they prove over, and in turn have no way to verify the usefulness of the proofs and function of the network as being sound, let alone altruistic.  As such, you can have no alternative but to assume the coin does not function.

The whole "big idea" of crypto-currency is that, by the combination of the details of the implementation of proof coordination and Satoshi's social contract, no-one need explicitly trust any other singular entity.