I know people are really fond of this talking point, but it doesn't hold up.
if you come to work at my sandwich shop, and you show up every day to make sandwiches, and then payday comes and I stiff you, then I have definitely stolen from you.
The strongest retort to this is that this would be an instance of fraud, since in the sandwich shop we probably came to an agreement beforehand that I would pay you e.g. $15 per hour, but in the case of IP, this is incomparable because we never agreed to anything. This is pretty straightforwardly addressed by the observation that we live in a society where IP laws are in effect, so regardless of how any one person feels about IP in the abstract, we're all bound by the rules that are known at the outset. Because your decision as the creator to move forward and create the thing was predicated on an understanding that the rules are such as they are, then I actually do have an obligation to observe them just the same as my obligation not to stiff you for making my sandwiches. I don't get to start a game of Parcheesi with you under the pretense that we're going to play fair and then reveal midway through that I won't be governed by all the rules on basis that I don't believe that they're entirely justified. Fairness cannot follow from false pretense.
The one way that it would be fair would be if I made my position known from the beginning and then everyone agreed to play anyway under my augmented ruleset, but it's important to observe that the obligation lies with me. What this doesn't mean, in a society with IP laws, is that it would be sufficient for me to declare (e.g. in a manifesto posted to my blog, or in an Internet message board comment, or whispered into the night) that I don't agree with IP and then proceed to fill up on all the stuff that I'd like. What proceeding with my augmented Parcheesi ruleset looks like is disengaging from the society where IP law is in effect and moving to Russia or Shenzhen or the habitat I've set up on the edge of Schiaparelli crater.