It was always possible that someone in the group of those sailors was affiliated with KGB, so nobody would ever say to the foreigners what they actually think. There would be also consequences for your family if one defects. In the end why would anyone leave your own country without having any prospects and social ties elsewhere. There were of course few naive people who managed to flee, but those were certainly not the stories with the happy end, once they realized that all these shiny bright colors were just empty shell that hides the ugliness of capitalism.