Yes, I do that, for multiple sites including this forum. Malicious hackers do not ask for permission, I don't perform anything intrusive / service impacting like DirBuster or another automated scanner without permission.
You do know that is both unethical as a pentester, and illegal. I guess you never took the correct security classes.
FYI:
http://www.google.com/about/appsecurity/reward-program/
https://www.facebook.com/whitehat
A "don't pentest our site without our permission" strategy is literally the worst you can have. It's as stupid as 'gun free zones' - if law abiding citizens can't bring guns, then only criminals who don't respect the sign will have guns. You're not going to get prosecuted if you perform responsible disclosure.
Like I said, I don't do intrusive pentests.