There wasn't any part of the essay which says you should start a startup, or that it is a morally valuable thing to do.
I somewhat agree with you that capitalism doesn't produce optimum value for society. Zynga's maybe an example of that -- I'm sure the are worse ones. But as the saying going, we have the worst system except for all the other ones that have been tried. For all the Zyngas there are some pretty good companies too.
Also, I think your question is essentially hypothetical or philosophical: "should be allow it to?" Who's we? Short of an overthrow of the US government, I think this segment of the economy will exist for a long time.
If you want to have an interesting reflection on capitalism, read "The Idea Factory", about Bell Labs. That is the other end of a spectrum -- a single company holding a monopoly for 50+ years. But it actually produced immeasurable value. It's interesting to think on which model produces more value -- a monopoly where people are free from competitive pressures, or an intensely competitive market.