Are those who use a product that has been leaked for free—while it is officially sold—considered thieves?

If the predominant probability is that the company owning the product distributes it for free to increase its spread, then there is no harm in using the free copies. The ruling of prohibition is set aside, acting upon the principle of the presumption of innocence from the rights of others, as Al-Izz ibn Abdus-Salam stated in Qawaid al-Ahkam: "Indeed, Allah created all His servants innocent of their obligations and bodies from His rights and the rights of His servants, until the causes of their obligation are realized."