Is scalping tickets harmful? IMHO, scalping tickets is no different from legitimate arbitrage unless manipulative.

Iirc, arbitrage increases surplus and hindering scalping is setting a price ceiling which leads to deadweight loss or something like that.

So why do some states ban scalping tickets?

I'm assuming that such states think there is some perceived harm to their economy. Strangely enough, why tickets? Why not bags, clothes or phones?
There is a good Planet Money episode on ticket scalping; I recommend it.

The reason for banning ticket scalping has nothing to do with economic harm, and everything to do with making the arts (or sports, whatever) accessible to people of more-modest means. Consider the fact that artists could, if they wanted, just auction off all the seats to their shows, capturing all the surplus and putting scalpers out of business.* 

If artists did this, however, lots of people wouldn't be able to afford to see popular artists perform. Many artists (the Planet Money episode uses Kid Rock as an example) want to try to make sure that their fans have a reasonable shot at attending, which means that they want to price the tickets low... but the only way this can work is if resale is illegal.

It's been suggested that artists (again, the same goes for sports teams) might benefit from making low-priced tickets available to fans for two reasons: because they plan on selling stuff to people once they're inside, and because an artist-fan relationship is hopefully not just a one-shot transaction. The idea is that fans who can attend the occasional show might be more likely to buy recordings, merchandise, and tickets in the future, providing the sort of long-running support that can keep an artist viable over many years.


  [P]erformers who undercharge their fans can paradoxically reap higher profits than those who maximize each ticket price. It’s a strategy similar to the one employed by ventures like casinos and cruise ships, which take a hit on admission prices but make their money once the customers are inside. Concert promoters can overcharge on everything from beer sales to T-shirts, and the benefits of low-priced tickets can accrue significantly over the years as loyal fans return.


*For the most part— there's some amount of resale that is sort of frictional, which arises from people not being able to attend a show and unloading tickets so they don't just eat the full cost, etc.