"if search engines were forced to disclose their algorithms and not just the signals they use, or, worse, if they had to use a standardized algorithm, spammers would certainly use that knowledge to game the system, making the results suspect."
Regardless of whether or not it would be a good idea for Google to be required to open their algorithm, I don't like this reason. It is security by obscurity. Spammers can only take advantage of flaws in Google's algorithm. By opening it to the public, I'm sure the algorithm itself would be improved.
The real reason Google doesn't want to expose it is that it is a competitive advantage. They should be upfront about that at least.