OK, and the thousands of other kinds of cancers are caused by viruses? 
When tobacco became the one stop boogeyman for cancer, a lot of research stopped.
In a nutshell, we know SOME things that are associated with cancer (radiation, asbestos, tobacco (likely 'hot' potassium from the soil), some other chemicals) enough to attribute those cancers to those things, which may well not be the only things that cause those cancers. I cant live in California, I'd be one big tumor (seems most everything causes cancer there if you read the labels).

We even credit some parts of the electromagnetic spectrum to cancer growth, but not others (sunshine, but not RFI from your cell phone?)
We have one hell of a lot to learn, and we don't know the full extent of the effect of viruses on human cell growth.
But with increasing research on viruses and cancer, we find that things like SV-40, a contaminant in many of the polio vaccines of my youth, is linked to cancer.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC452549/ From journal article :
Recent studies have shown that primary human mesothelial cells respond to SV40 very differently from fibroblasts; the mesothelial cells are highly susceptible to SV40 infection and transformation.
Which may well mean that at least some of the Mesothelioma out there has a cause that has been mis-attributed to something other than a virus, a once common vaccine contaminant, or both may be a factor in Mesothelioma. This is an aspect of cancer research and viral research that would have been a far better use of our taxpayer dollars than creating gain of function viral chimeras to unleash on the world.
But the bottom line is that instead of focusing on research, an awful lot of the money spent on cancer is spent on oncology (treatment/management) and "prevention" which means tons of dollars dumped into commercials like the "Cigarette Mash" in an effort to impose behavioural changes, but the research into causes has traditionally stopped at the first thing found, and does not necessarily present a complete picture.
Instead of attacking "second hand" or even "third hand smoke", perhaps other investigations could be undertaken to look for a causative pathogen. But then, it seems as if the research (grant or contribution based, including the Government) is aimed more at protecting or looting certain deep pockets than it is aimed at finding the answer.
YMMV, but do read the article I linked.
I am interested because a number of people I knew in my childhood have all died of the same form of cancer. I left that area at 17, they lived there all their lives, but the foods we ate, the exposures to chemicals we had were all very similar, and the question in my mind, aside from a genetic link between four of them, is whether we were all exposed to the same factors via whatever vector, be it chemical or pathogen or both working synergetically. We all ate seafood from the same estuary and wild game from that area as a large part of our diets, but worked in different jobs. So the question for me is one of what common factors we were exposed to which may have caused the cancer. We all got the Polio vaccine at the same time, so SV-40 is on the radar, too.
In short, we don't know. We (meaning the scientific research community, not me personally) have found the mechanisms by which viruses cause some cancers, but do not know the full extent of that. Other causative factors apply, such as 1/30,000,000 of a gram of weapons grade plutonium inhaled is a sure case of lung cancer--we know radiation can be a cause.
We believe that certain irritants (physical, radiological, or chemical) are a cause, but they may just exacerbate a condition which lets cells grow out of control by persistent irritation, causing repeated cell damage and making an environment where that short-circuited control mechanism is more likely to surface as as cancer, as opposed to more normal cell damage, death, and replacement, where the ordinary mechanisms which keep those processes regulated function normally.
This is complicated by the fact that not all viral infections, even of the same virus, lead to cancer, only certain strains of those viruses, which is why of the over 100 HPV strains out there only 14 are linked to cancer.
For those who want to prevent cancer, to find its causes, there is a lot of research yet to be done.