In the UK at least, the Consumer Protection from Unfair Trading Regulations of 2008 imply that it is an illegal sales technique. There has been some success in courts prosecuting this to a degree that most subscription services will also display the full price, less they become the example case.
There are. In fact, they were strengthened significantly quite recently, particularly in the area of online sales, and now with potentially quite serious penalties for any business that doesn't comply.
If you're interested, the main underlying EU rules are in Directive 2011/83/EU.
