Most people has a tendency to inaction. If you give users two services of equal worth, but in one you need to unsubscribe to get rid of it and the other the default action of doing nothing results in unsubscribe, more people are going to end up paying the first one because they forgot or did not care enough to go and unsubscribe. Scammers who sends false bills use the same concept, as they simply hope some people will just automatically pay and not think or even read what the bill is.
In the end, using a credit card trial where the user need to actively unsubscribe at the end of the trial will give you more users who dislike your product or simply isnt using it at all. However, hopefully its enough of a barrier so people who wish to have a "viral" product can't use it.