It is my impression that the fact that "scalping tickets" is considered illegal (or at least restricted) in many parts of the world, may be due to the following reasons:
A) Transactional: A ticket has a consumer price printed on it. This means that the supplier of the service has announced/committed to a price at which he is willing to provide the service/product. This creates a different transactional framework than the one where by design a market works auction-style, or bargaining-style. In many places, re-selling such products at prices higher than the printed one, is legally considered a violation of consumer rights, even if only indirectly, because, in such cases one has to at least clearly disclose also the nominal price (i.e. the seller should "shout" something like "I sell a ticket that has a nominal value of 10 USD for 13 USD "). Have you ever heard such an announcement?
B) Tax: In many cases, ticket scalpers are not official wholesalers (who in any case would have bought the tickets at prices lower than the nominal one, and then would resell it at the nominal price), but rather, undeclared entrepreneurs buying tickets at the nominal price as though they were consumers, and counting on excess demand to sell them at higher prices in under-the-counter transactions.
C) Ethical: While event-going cannot be considered life-critical, it does have a strong element of "psychological/emotional" (i.e. not-rational) desire. When one counts on such an aspect to sell at a price higher than the actual supplier of the object/service of desire demands, it is more often than not deemed as "exploitation", in many cultural settings. While from the point of view of Economics, this is just market-clearing, we should not forget that how Economics views the world is not necessarily how societal ethics (or ideals), do: Although no third party is forcing an event-goer to go to the event, societies tend to consider a buyer driven by such desires as "having the right to be protected from any negative side-effects of his own desires" -and themeselves as having the obligation to provide such protection.
An interesting article/review of the matter, with some examples regarding anti-ticket-scalping regulation in the USA can be found here. The article discusses also the underlying worries/views that appear to lead to such rules and regulations.
It appears that ticket-scalping is treated as a special case of scalping, and has its own legislation.