You can never trust an open wlan. For example: I run a (multi homed) proxy on my lan with a optional redirecting honeypot. It logs all trafic.
You would never know.
FYI: I have no bad intentions, I just want to know what is going on on my lan.
(I could use the honeypot to get back at you if I wanted...)

But I agree, 99% of all open wlans are safe.