Buying a bunch of different tickets to different shows you intend to go to, and selling some of them because things come up or you change your mind, is one thing.
Buying 20 tickets to the same event that you couldn't care less about and selling them for cash outside the venue is something else entirely.
Yeah, scalping provides very little value to society except profit to the scalpers. It's parasitic. Unfortunately the methods to combat it, like ticket lotteries, better identity verification and per-id quotas for purchase and transfer, aren't in the interest of ticket vendors. Artists generally can't wield much power either.