"i set up a wireless access point that they could use, set to give everyone permission to use it, but arrest them anyway because i'm an idiot" - hardly a good argument
if a person puts all their belongings out in the street and sticks a sign on them saying "Take whatever you want" and hires a bloke to walk around the streets shouting to everyone "FREE STUFF, YOU CAN TAKE ANYTHING YOU WANT" then you go and accept their invitation and take some of the property they gave to you, you are a thief? "i did give you it, but at the time i didn't know it was worth money so you're a thief for taking something i gave to you"
if you are granted permission, you are granted permission - they set the thing up to give you access, they chose the option of granting you explicit permission, coming out with "well when i gave you permission i didn't know what i was doing" does not change the fact that they gave that permission and therefore you did not do anything wrong, as using it with the owners permission is clearly not wrong! (this is going on the assumption they were able to either disable the wifi, or enable some form of encryption)
now if they had enabled WEP and you had gone and broken the WEP key, then this is a different matter - because although it is not secure, it is also not an open invitation granting you access, and you only got access using forged credentials
it is a clear thing, access points have a way of indicating if they are public open networks that you are allowed to use, if the owner uses that system to inform me that it is indeed such a network, then i have the owners permission to connect - if i did not have permission then i would not be receiving broadcasts from the thing telling me that i do have permission, and additionally i would not be receiving replies to my connection requests confirming that yes, I "MAC Address xyz" do in fact have permission to connect and use it
of course things like an access point broadcasting it is a public open access point, then using MAC Filtering to only respond to certain MAC Addresses is another issue and is not clear cut as it is a muddled mix of both things - thankfully i have only seen a few such networks (I assume that one of the devices using it has problems with WEP/WPA? only reason I can think of...) - in that case I think it should be considered that as it is being broadcast as public and open it is free to use, as a private network would have such public broadcasting disabled at the very least, and a network which does not broadcast that it is open to all, and which only responds to connection requests for selected devices, is not an open access point (even if no encryption is used)