"Unwritten rules" are as varied as personalities. Some motorcycle riders won't wave to moped riders. Some sportbike riders won't wave at cruiser riders (and vice versa). Some riders of customs won't wave at riders of stock motorcycles. Some HOG riders won't wave at any non-HOG, and the list goes on forever.
Personality is a big part of it. A generally friendly guy is happy to get a wave from anyone and will wave back. There is usually a reason (sometimes a good one) for giving the cold shoulder. A rider who paid $20,000 for 1550 coveted cc's, and who looks like he's right out of 'BadAss Bikers' magazine may look down on a guy on a $4,000 600cc Honda cruiser wearing a fullface helmet that doesn't even match the bike. Or not...
People who invest more (time, money, whatever) in something usually expect some exclusivity that seperates them from the crowd, otherwise there's no point.
A motocop who has had extensive training may look down on other riders as inferior riders (which they likely are)... or he may simply not want to give an impression that he is fraternizing. Maybe he's worried that other motorists will see a friendly gesture toward another motorcyclist as 'favoratism' or whatever.