You are right, but the newly revealed indices suggests that your personal data are not collected primarily from the cloud providers after they arrived to the destination (like the spy agency would enter the Gmail repository through a backdoor and vacuum out all the data), rather, the whole (or most of) Internet traffic is monitored on the main routers and everything collected. Only thing which is not yet clear (at least to my knowledge), how much is this being done outside of U.S.
From this perspective, boycotting US based cloud providers is probably a reasonable thing to do, but it is not clear if this will results in less of your personal data being collected, if you just switch the provider. For example, I noticed it is now popular to switch from Gmail to other providers like fastmail.fm - I'm really not sure if this would be helpful at all.