Yes, copyright/patent infringement (depending on the exact details). But there are circumstances that might mitigate how the public (and perhaps law) feels abo

Yes, copyright/patent infringement (depending on the exact details). But there are circumstances that might mitigate how the public (and perhaps law) feels about that, eg, if company B bought a copy of the software and slightly modified it to work on a new operating sysem, then sold just the changed version.