Normally we agree, but I think we are in disagreement this time, Tim.
It's a case of correlation but no causation. Sunblock becomes popular and at the same time skin cancer incidents go up. Must be some causation, huh? Sort of, but not the way you think. Here are my several theories as to why this correlation is present.
A). Sunblock requires proper usage to work. How many people don't put it on their entire exposed body? How many people put on some SPF 15 and then forget about it the rest of the day? How many put on sunblock and then go swimming and never reapply?
B). People forget. How many people just forget to slap on the sunblock at all? I know I forget all the time. I have the kind of skin that doesn't burn easily*, so I just forget about it. (*Just because I don't burn doesn't mean I tan. I have some real funky skin issue that prevents either from happening... my mother's fault :p )
C). Skin cancer, like all cancers, has come into the public spotlight. In the 70s I severely doubt it was in the public's eye the way it is now. That is the problem with all the cancer data that shows a significant rise in cases. It's mostly because back in the day people just didn't know/bother with cancer. They died, and that's it. Doctors, often, didn't even recognize it. My mom, a nurse, even saw this change in awareness take place. When she first started as a nurse (a long long time ago...) cancer was almost rare. Now they have a fully staffed cancer section that people come to from all over the region.