No, I don't believe they 'owe' it to the neighbors. Are they stoned when the kids are around? A different story, but not the question you are asking.
For what it's worth, I do think that all parents have the job of 'vetting' who their child is spending time with. Would I leave my kid with someone who was drunk, overly medicated or high on anything? No. Mentally ill? No. I see bigger issues at play. Does someone have to notify the neighbors if they have bipolar disorder*, which would likely have more of a negative affect on those around them if not managed correctly? Where does privacy and personal responsibility end and public 'concern' begin?
I think the presumption that we should all notify our neighbors about our personal proclivities is a bit intrusive, to say the least.
Are you planning on leaving your children in their care?
If you ask them "do you smoke pot?", if that's your concern, then that's your right to do so. Just don't be surprised if they tell you that it isn't any of your business. It isn't.
For what it's worth, I keep my friendships based on the kind of person they are on an everyday basis. I know plenty of sober people I would never be friends with in a million years. Over the years, I've discovered that a jerk is a jerk, drunk or sober. Same with pot. Finding out someone smokes pot is not a big deal for me. I just expect that they won't do it in front of my kids or act inappropriately. The same thing I'd expect if they didn't.
(*please know I'm not trying to demean anyone with bipolar disorder. However, from my experience, I've seen a lot more trouble in my own family with unmanaged mental illness than I ever did with the people who wanted to have a toke or two in the evenings, after the kids were in bed.)