It is, if you are leveraging one monopoly to attempt to gain more. This is what happened with Microsoft and Internet Explorer (via the Windows monopoly)[1]
Had Microsoft merely provided advertised IE with Windows there never would have been a suit. Even bundling alone would have been a hard case to make. They did far more than just ship IE with Windows.
1: https://en.wikipedia.org/wiki/United_States_v._Microsoft_Cor....