" Because if we knew of the existence of a patent then it could be shown that we were knowingly infringing on the patent."
This legal advice was been valid at one point.
However, nowadays, willful infringement requires more than just knowledge.
It has for a few years, but the most recent supreme court decision also strongly supports this.
See Halo Electronics v. Pulse electronics (http://www.supremecourt.gov/opinions/15pdf/14-1513_db8e.pdf)
in the concurrence:
First, the Court’s references to “willful misconduct” do
not mean that a court may award enhanced damages
simply because the evidence shows that the infringer
knew about the patent and nothing more.
Second, also note:
“failure of an infringer to obtain the
advice of counsel . . . may not be used to prove that the
accused infringer wilfully infringed.”