[njharman]

They are creating monopolies (cartels) and creating artificial demand by cornering the market. Things most capitalist systems regulate/limit.

They are speculators. People have instinctive dislike for speculators as they produce nothing and in scalpers case provide no benefit (such as liquidity) to society.

[namdnay]

Scalpers do provide a benefit though: If you have more money than time, scalpers enable you to easily buy the best tickets, those which would probably have been sold out within a few hours if everyone could buy them at the artificially low prices set by the venue.

Speculation is just another word for betting, I don't think that's an inherently bad thing.