Earlier quoted context omitted.
It can be, if the company that does it has a monopoly in the market of operating systems, which would give that company an unfair advantage in the market of browsers. http://en.wikipedia.org/wiki/United_States_v._Microsoft_Corp .
Poor wording for my part, it must say "it is not an anti competitive practice per se"
> Doing an anti competitive practice is legal if you don't have a monopoly?
A practice being anti-competitive (in a legal sense) depends on the act being done by a company that is a monopoly, or part of an oligopoly.
For example, a company giving their product away for free is not illegal per se. Say, as part of a promotion, in order to get their product in consumer's hands instead of a competitor's product. In a healthy market, this is a valid way to do business.
But, if the company that decides to give their product away for free is a monopoly, and the result of that act is squashing what little competition might exist or is entering the market, then this same act (giving the product away for free) is illegal.