1) I take something away from you. You have less of it as a result. Copying is not theft.
2) I deprive you of something, such as exclusive use of your land (e.g. by trespassing) or failing to follow through on a contract. Copying is theft.
Both of those are used by different communities, who both become angry at the other.
This is a semantic argument. Most members of both groups believe that there are times when copying is wrong, and are split on when that is.
However, to group #1, "stealing" and "theft" is a highly offensive term. It's much like saying "You raped me up the ___ when you didn't pay my contractor bill on time" or other hyperboles. Not paying my bill was wrong, but it also wasn't rape. It devalues rape, insults you, and is imprecise. You should use the precise "copyright violation" which describes exactly what happened.
To group #2, NOT calling it theft is offensive, since it devalues the costs to businesses and creators of copyright violations. Whether you agree with them or not, they have certain rights under the law, and picking-and-choosing which laws to follow is wrong (especially when it's self-serving).
Because the two groups mean different things by the same words, they can never hold a rational conversation with each other, and become offended when they hear the other group speak. It's how we polarize. It's unfortunate, since there's an important discussion to be had about the limits and enforcement of copyright and patents, which really should start with the copyright clause in the constitution, and when it helps versus impedes progress and economic growth. That's a discussion possible to have analytically and rationally.