"Intellectual property" isn't a thing, "Intellectual Property Rights" are. They're not "antithetical to free market principles" if you accept that the "free market" requires regulation and that there are benefits to society by the cost of temporarily restricting free use with the subsequent benefit of ideas and expressions not being lost on the death/loss/bankruptcy etc of the IPR holder.

IPRs are (in general) things that are protected by law and open to licensing and civil suits for damages if used outside those laws and licenses:

* Trade Secrets and NDAs

* Copyrights

* Patents

* Trademarks