Why you should not buy from Ticketmaster.
The main reason to refuse to buy from Ticketmaster is that it demands customers pay digitally, and thus not anonymously. I am pretty sure that purchasing a ticket through Ticketmaster also requires running nonfree JavaScript software. (Can anyone send me factual confirmation of this?)
However, there are also reasons to boycott Ticketmaster:
I have never thought of attending a concert which used Ticketmaster. The concerts that interest me tend to smaller, with kinds of music that are less well known. You can buy tickets locally easily for those.
Research sourced from stallman.org.