Anti-big corporates and monopolies.

If you're for a free market, ensuring large corporations can't price-out the competition with unfair practices, political influence, etc. is key. One of the few roles of government should be ensuring a free market.

Basic economics suggests that a free market exists when certain criteria are met. For example, low barrier to entry into the economy. If, as a corporation, I make entry into the economy and marketplace difficult with my political power, price fixing, etc. then the market no longer becomes free and efficient. It becomes monopolistic, inefficient, and toxic and the externalities go through the roof because there is no ability for the market to adjust. We've seen this in almost every single industry that has any value in modern day. The difference between large corporations and the government has become so difficult to define that they have essentially melded into one thing.

Pro-free market libertarianism advocates that this is a very, very bad thing and that the role of government would be to self-regulate (to get money out of politics) and ensure that the market stays free.