I agree Sam may be in an especially good position to make guesses about the future of companies, but that doesn't explain where the investment money comes from.
I'm assuming the same for Kevin Rose who's a pretty active angel.
Likely the sums invested are fairly small as well.
The Foursquare guys took $4M home out of a current funding round
http://www.theequitykicker.com/2010/07/15/foursquare-founder...
"income exceeding $200,000 in each of the two most recent years or joint income with a spouse exceeding $300,000 for those years and a reasonable expectation of the same income level in the current year"
And it is not like you have to submit to an audit to the company before sending them a check -- all you have to do is sign something that says you are accredited. That way you will have a harder time suing the company claiming you were mis-led or whatever.