Concerning Lying as a Cultural Difference

I had a reading assignment for my Cultural Anthropology class. As part of it, I read Carol McKinney's Globe Trotting in Sandals.
In one chapter, McKinney talks about lying and about how and why it sometimes happens. I don’t know if I liked some of the things she said, or, rather, the way she said it. When she spoke of natives lying, she said that they don’t mean to deceive but they’re constricted by their culture. An example would be a missionary asks a native if they can have an interview tomorrow at 7 and the native says yes, not because they'll actually show up but because it would be rude to say "no." So, it's because of thir culture that they have to say no. This sounds like a cop out to me. “I can’t help sinning, my culture made me!”
McKinney also said that what might be lying in Western culture might not be in other cultures, because in their culture it might be that it’s a lie if you get caught. I don’t like this at all. It sounds as if she’s making lying relative, which would make right and wrong relative. And that is not the case.
I think that some cultural differences are acceptable, like methods of worship. But I don't see this as a something missionaries need to come to understand and tolerate. Of course, we can ask questions more wisely, asking, "can we meet tomorrow at 7 or would is there a better time for you?" so that they don't have to say "no." But we shouldn't tolerate their sin and justify it as a cultural difference.
(Of course, McKinney may not be saying this and I may be taking it the wrong way, but this is the way I see the subject.)