I'm confused by the forbidden surveillance example. Generally surveillance camera's are legal for any place where there is no expectation of privacy. The expectation of privacy is largely only ones home, outside of that you can be video taped all day long by anyone. I'm not sure how this is analgous..

The million messages example is interesting. Though, what examples are there? In what cases is something legal to do it once, but there is some threshold where you cannot do it many times?

The "sending millions of messages" is only perhaps illegal because it breaks terms of service. Or, the one message is perhaps also illegal but nobody cares to pursue litigation for one instance of an infraction. The point remains though, if an individual does something once that is legal - it makes that activity legal, period and full stop. No?