"Free markets" as described in mainstream political discourse only represents freedom for a small segment of society: industrialists and banksters. Since capitalism depends on a power imbalance, "free markets" gives even more power to the rich, which automatically results in worsened conditions for the rest of us. Opposition to free market neo-liberalism and globalization is really about opposition to corporations, tyrannical regimes, colonialism and ecocide.
All anarchists are not necessarily opposed to free markets as a concept. From mutualist.org:
"Mutualists belong to a non-collectivist segment of anarchists. Although we favor democratic control when collective action is required by the nature of production and other cooperative endeavors, we do not favor collectivism as an ideal in itself. We are not opposed to money or exchange. We believe in private property, so long as it is based on personal occupancy and use. We favor a society in which all relationships and transactions are non-coercive, and based on voluntary cooperation, free exchange, or mutual aid. The "market," in the sense of exchanges of labor between producers, is a profoundly humanizing and liberating concept. What we oppose is the conventional understanding of markets, as the idea has been coopted and corrupted by state capitalism.
Our ultimate vision is of a society in which the economy is organized around free market exchange between producers, and production is carried out mainly by self-employed artisans and farmers, small producers' cooperatives, worker-controlled large enterprises, and consumers' cooperatives."