I also don't believe in IP, but my career is literally in IP (I work with patents, within the IP division of a large company). IP is a game -- it has a set of rules that people play by -- and my job is to work with the rules as they are implemented, not to worry about whether or not the rules are good rules (or whether or not they accurately reflect some aspect of reality).
Refusing to work with IP because you don't believe in it is like refusing to play WoW because you don't believe in wizards.
Now, if you have a philosophical objection to IP, that's a different issue. You need to determine for yourself what your philosophical objection is worth in monetary terms. Will this company be able to pay you enough to make you temporarily set aside your moral standing (or, alternately, will the release of this software produce more good in the world than its licensing produced bad in the world)? If the answer is no, then leave.
The answer, unless you are already wealthy, is probably yes -- few people are willing to starve to death for their position on the morality of IP, and if you quit your job and refuse to work for any company that uses or produces proprietary products, you will probably run out of money and starve to death on the street before you find a new job that fits your criteria.