Today online security is a very important factor. Many businesses are completely based online, and there is tons of sensitive data available to check out only by using your web
Informing the administrator is the best thing to do, but some companies just won't take unsolicited advice. They don't trust or don't believe the source.
Some people would advise you to exploit the security flaw in a damaging way to draw their attention to the danger, but I would recommend against this, and it's possible that you could have serious consequences because of this.
Basically if you've informed them it's no longer your problem (not that it ever was in the first place).
Another way to ensure you get their attention is to provide specific steps as to how it can be exploited. That way it will be easier for whomever recieves the email to verify it, and pass it on to the right people.
But at the end of the line, you owe them nothing, so anything you choose to do is sticking your neck out.
Also, you could even create a new email address for yourself to use to alert the websites, because as you mentioned, some places it would be illegal to even verify the exploit, and some companies would choose to go after you instead of the security flaw.