The fact is, the sun on any day of the year can cause changes to the cell structure of the skin that can lead to various forms of cancer, including malignant ones that spread through the body and kill people. To suggest that even in summer the risks are something we worry too much about is errant and dangerous.
The fact also is that skin cancers resulting from sun exposure don't appear overnight. They appear often years after constant exposure has stopped. So people feel confident of remaining in the sun, getting sunburnt or deeply tanned, with no obvious ill effects from cancerous changes to moles and other parts of the skin.
The tanning industry in Victoria, Australia, has almost gone out of business in the past six months after a woman claimed to have contracted skin cancer from using tanning beds. She died, but made sure the world knew of her plight and what she believed caused it beforehand.
The Vitamin D issue has some credence, but more because, I believe, people spend so much time indoors working, or spending their leisure time watching television or playing on computers, rather than occupying themselves with outdoor recreation.
And the exposure requirements to the sun for adequate Vitamin D intake are far less than would result in skin damage that may lead to skin cancer.
The only saving grace about winter sun is that because the earth tilts on its axis, the radiation has to penetrate a thicker layer of the atmosphere, filtering out some of the UV. The days are also shorter. Pollution, such as the thick clouds that hang over cities such as LA also can help, but carry their own health problems.
And just as a little heads up... many, many of the skins cancers that are routinely burned off people are on the nose, cheeks, temples, lips and ears... the very places that continue to be exposed to the sun even in winter. And when there is snow and ice, the sun's radiation is reflected off that as well, intensfying what's already being taken in directly from the sun. Windburn after a day on the snow is not... it is sunburn.
As to the OP's initial observation about the "hot summer sun" and "tanning", people who ride bikes and apply sunscreen are more likely to remain comfortably "cooler" and not perceive the heat of the sun as the day passes.
By the way, have you noticed just how awful those old people with deep, deep tans look because of the wrinkled skin? Old habits die hard and you can expected those wrinkles to have come from skin degradation resulting from sun worship. Chances are there are skin cancers forming there, too.