Doesn't whitelisting amount to closing of interface? They may very well use the CalDav protocol which is open, but how good is it if the consumption interface is not open?
Perhaps it was done to prevent spam, or to prevent misbehaving clients from wreaking havoc on its servers.
From hearsay, I understand it's reasonably easy to get on the whitelist if you're a moderately-known app developer or a legit institution. This would lead me to believe Google's reasons are less nefarious in this case.
