Economist Luigi Zingales provides a helpful explanation on the difference between being pro-market and pro-business:

A pro-market strategy rejects subsidies not only because they’re a waste of taxpayers’ money but also because they prop up inefficient firms, delaying the entry of new and more petitors. For every “zombie” firm that survives because of government assistance, several innovative start-ups don’t get the chance to be born. Subsidies, then, hurt taxpayers twice. . . .

And a pro-market approach panies financially accountable for their mistakes—an essential policy if free markets are to produce sound decisions. A pro-market party will fight tirelessly against letting firms e so big that they cannot be allowed to fail, since such firms may take risks that panies would never dream of. . . .

[The Republican Party]has to move from a pro-business strategy that defends the interests of panies to a pro-marketstrategy that fosters petition and freedom of entry. While the two agendas sometimes coincide—as in the case of protecting property rights—they are often at odds. Established firms are threatened petition and frequently use their political muscle to restrict new entries into their industry, strengthening their positions but putting their customers at a disadvantage.

(Via: AEI Ideas)