Of course you are right, but if you have no time, lake some experience and own some extra cash, you may prefer to buy an EA instead of building, and that's why you should be cautious if you took that decision!
I think that you should never purchase an EA where the author won't allow you to see the source code. It is SO important to use good money management that you should not ever leave this to anyone else to decide. Its really easy to loose money; so hard to make it!