It wasn't too long ago that scalping, i.e. reselling tickets, was illegal. The law was changed to make it easier for fans to resell tickets that they couldn't use. But now people are taking advantage of the situation to make money.
It's a complex problem. Some say the problem is that the tickets are too expensive, but if the price is too low, then it actually gives the ticket holder more incentive to sell because the profit from reselling becomes larger.
Most people are going to do whatever benefits them the most, so we can't rely on ticket holders "doing the right thing." But we all hate to see the stadium taken over by opposing fans, so I think we do want the team to step in to try to implement reasonable measures to prevent resales.