(I left out bits that are not relevant for this post).Permanent right of residence
15.—(1) The following persons shall acquire the right to reside in the United Kingdom permanently—
(c)a worker or self-employed person who has ceased activity;.
(d)the family member of a worker or self-employed person who has ceased activity;.
So what about the following scenario: as an EEA national you find a job in the UK, you work there for let's say a year or so, then quit your job and spend your time enjoying daytime television instead. And as a result you and your family members have acquired PR, and you send in your EEA3 and EEA4 forms to get confirmation.
And of course, after having acquired PR nothing stops you to engage in any economic activity, for example finding a new job.
I am fairly sure that the EU directive has such provision to protect people who are sacked or (involuntarily) retire, but it seems to me that the UK implementation as quoted above allows any reason for ceasing activity, voluntarily or not, setting the door wide open for abuse.