They were invited to provide some kind of assistance, but I can't imagine they were given the green light to upload arbitrary code into the countrywide telephon

They were invited to provide some kind of assistance, but I can't imagine they were given the green light to upload arbitrary code into the countrywide telephone exchange without notifying the Greeks at all, or providing a copy of it. It's not just that they added Greek officials phones to the monitoring list alongside legitimate suspects, but the whole program itself was hidden. Nor is it clear how the US could have forwarded any legitimate threats captured without admitting the whole program. The whole Olympics rationale is a bit weird too. Yes, it is a big event and there have been a few terrorist attacks in its history, but relatively few considering its huge scale. Also, almost every state actor is participating and thus unlikely to cause any major trouble. It's a big propaganda thing for most of the US-order enemies. It doesn't really make sense why the NSA would have been authorized such extensive access, to the exclusion of the host country's own personnel, instead of requesting help from neighboring EU countries that already presumably cooperate on law enforcement. That sounds like a cover story the US made up to change it from "act of war" level to "oops we forgot to ask if this was okay."