A better analogy would be drawing up plans from his yacht and using them to build another yacht for yourself. This is illegal, but it would be silly to claim that it is "stealing" from the yacht manufacturer.

It's stealing because you're depriving someone of that yacht. It's not a lost sale because you wouldn't have bought one.

It’s not really important whether it’s ‘theft’ or not, that’s just not relevant to the question whether it is morally acceptable or not.