Exactly how is it cheating? As long as the service clearly states upfront that your card will be charged at the end of your trial, it's the user's decision whether or not to proceed.
I'd argue that it would add some amount of inconvenience to the user, and lower conversion rates as a result, if the user has to pay to continue using the service.
put it this way - if you asked users up-front "when your trial expires, should we convert you to a paying customer and automatically start billing you?", what fraction of them would say "yes"? i'd guess it's pretty low, and most companies know it. the fact that they're deliberately taking advantage of people's tendency to forget to cancel counts as cheating in my book.
I'd argue that it would add some amount of inconvenience to the user, and lower conversion rates as a result, if the user has to pay to continue using the service.