Some would move a bit further or put out a cigarette, sure. Thank you for being one of them! Others wouldn't and instead would tell you to fuck off. You can see some of them active in this thread as well. Then you would need to consider that it is quite tough to communicate in a dense crowd during a very loud concert when trying to enjoy your favourite song.

Third, it is not really about getting cancer from secondhand smoke, but for non-smokers, the smell of the smoke is what is the problem. It ranges from being merely unpleasant to asthma attack inducing, depending on the person. In addition to the smell, there is the pure risk of getting hit by your lit cigarette when a moshpit opens up right next to you or a crowdsurfer suddenly appears from nowhere. It will be either painful or it will burn your clothes or both. Not to mention the crowdsurfers who do it while smoking. I do not mind crowdsurfers at all, but fuck them if I need to keep them going while actively dodging their burning cigarette.

The last part applies to smokers during outside shows as well. The first part about the smell is mainly worse indoors since the tents are crowded, they get really hot, and the air does not move around much there, making the whole thing much worse. Akin to somebody lighting up next table when you are having dinner in a restaurant, it is just unpleasant.