If buying = owning then pirating* = stealing, because you own it without buying.

This isn't the point being made, and I think why you think it's illogical

Theft requires you deprive someone of an item, not that you get something without buying it. If your definition of theft were accurate then getting a free game would be piracy, which is silly

UNLESS - we examine what "stealing" is

Theft of a persons property without intent to return. Legally piracy and theft are different, not just semantically. There's no discussion to be had about what stealing is as it's not what's happening