Because atheists believe that over millions of years of evolution, over all this time using the sun tanning powers, the monkey became a human. And by the way, atheists also believe that eventually from the monkey to human, human is very slowly becoming a superhuman.
Evolving, ever evolving. Maybe it’ll grow some wings or something. Because of the environment, right? But if they use sunscreen, that is basically defeating the purpose of evolution.
They are disturbing the atheistic, sacred, holy process of evolution by preventing the skin from evolving using sun tan. Atheists either stop using sunscreen or accept and believe that there is no God but Allah Azzawajal and he made man with his hands. Or just believe that the sun rays are beneficial.
Take your pick, atheists. There you go. You are in a conundrum now.
Islamic Principles Website 