Intellectual property is a misnomer, or at least a weaker form of property. It's not about free speech, but rather about the particulars of owning an idea which may reside in another mind. Calling it theft outright is framing the question. Of course people are against theft, but if their ideas of theft are different, they are referring to different things.
Furthermore, there are three distinct cases where IP is understood differently: trademarks, copyright and patents (I suspect you are referring to copyrights). Some people may be against only one or two forms of IP, while accepting the others.
A form of copyright on a collection of facts. In the U.S., the famous "telephone books aren't copyrightable" case (Feist v. Rural) held that mere compilations of facts aren't copyrightable, because they aren't a creative work, and copyright is intended to protect only original, creative works. Collections with some degree of creativity in the selection and arrangement might count, but an alphabetical listing of all people in an area code with their phone numbers was held not to possess any creativity.
Other countries have more of a "sweat of the brow" view of copyright, that it's intended to protect any work that took effort to produce, so even a compilation of mere facts could be copyrightable, if it took a lot of effort to compile them. The EU now has a separate pseudo-copyright for databases that wouldn't otherwise be copyrightable (http://en.wikipedia.org/wiki/Database_Directive).
The main people pushing that are, as you might guess, those who have amassed a lot of data that isn't protected by normal copyright. For example, stock-ticker data is not copyrightable (it's just a mechanical listing of facts), and the companies that make a lot of money licensing it aren't happy about that.
Furthermore, there are three distinct cases where IP is understood differently: trademarks, copyright and patents (I suspect you are referring to copyrights). Some people may be against only one or two forms of IP, while accepting the others.