How do you know that they have not been penalised? Google may just be ignoring the hidden text and they rank well for other reasons.

I have seen a lot of sites with hidden text in Google - for almost all of them, I fail to see how they can be benefiting from doing it.

Google don't manaully remove sites as the result of spam reports (unless it something like a porn site taking over a Disney site). They use the spam reports to learn about spam techniques so that they can detect them via the algorithm and automatically penalise them; periodically recheck them to see if cleaned up; make them serve time in the 'penalty box'; then reinstate.

I guess Google get a huge number of spam reports, many of which may not be legitimate (ie S.P.A.M. = Sites Poistioned Above Mine).

CBP