It's hardly stealing if the original owners still have a copy. It's more of a breach of terms of service, or unlicensed access to intellectual property.
The "theft" spin on copyright violation is the result of years of lobbying by the media industry because they foresaw a drop in sales when people didn't need to buy the same thing over and over again.
Property is about who has the right to control something, not about scarcity. Although its original application was to a physical object, it's not limited to physical objects. So, "stealing" can be considered in both a narrow context and a broader context. The narrow meaning refers to literally taking a physical object away from someone else and giving it to yourself. But the broader meaning is wresting exclusive control of the property from the lawful owner. So, in that sense, copyright infringement and license violations can be legally thought of as theft.
The "theft" spin on copyright violation is the result of years of lobbying by the media industry because they foresaw a drop in sales when people didn't need to buy the same thing over and over again.