I thought it was that anti-competitive actions were illegal (or just heavily frowned upon?) and monopolies in and of themselves aren't bad, just if they abuse their power in an anti-competitive way.
IIRC, it's basically has to be the combinations of the two. Monopolies aren't illegal in and of themselves. But neither are actions that may be deemed anti-competitive. It's only illegal when a monopoly (or a dominant player - doesn't have to be a monopoly) exploits its position to gain further advantage through anti-competitive actions.