Because one of those has the ability to arrest you and the other doesn't?

Sure, but we've got a pretty spiffy legal system that protects you from that, and in any case the NSA has no incentive to arrest me. I have no real protection from Google or its employees selling my information, using it against me, etc, and as a practical matter they have a lot more incentive to do that than the government does to arrest me.

Funny - the way I look at it, the question is, why aren't people worried about Google, et. al. monitoring you the same way the NSA does?

You can opt out of google's monitoring. Can you opt out of the NSA's monitoring? Do they even pretend?

You can opt out to the extent that Google allows and to the extent that Google is transparent about it.