SB, you can't possibly be serious! If someone leaves their front door open and you walk in and help yourself to the contents of their house, you seriously haven't committed any crime..?
you will be stealing someone elses time surely ?? paid for etc..you could also be downloading improper material...and let this other person suffer any consequences...don't risk it !
the whole reason for encrypting your access point is to secure it. If it is unsecured you cannot stop someone from using it. It could be someone stood outside of your house with an iphone. As for downloading dodgy content the police would soon work out that it wasn't the router owner that did it.