My opinion fwiw- TPL / Alliacense (and by default PTSC) are staying under the radar because of the current uncertain legislative/political patent climate out there.  There are movements afoot to change the way patents are attained, retained and enforced.  Any fundamental change in the patent system can have serious consequences to every company involved in the infringement business-which would include  infringers, patent holders and enforcers like Alliacense etc.

TPL/Alliacense do not want to be drawing attention to themselves with large and LOUD settlements with some of the biggest companies in the world for the simple reason that this kind of thing could be spun to be the exact reason why patent reform is needed. 

Most of us know the long tangled story about why the IP was not monetized to any great extent up until a few years ago here, but in the hands of some crafty PR people from the infringement "community" the TPL's business model could be slanted in such a way as to put the infringers in the position of being victims in all of this with TPL / Allicense as the evil enforcer of "obscure" patents held since the 1980's.

There have been few American companies (HP & Lexmark and they are pro IP) out of the 30+ who have licensed or "settled".  Does anyone think it is a coincidence that TPL / Alliacense are not going after American companies yet?