Q Why is it an opt-out system?
A Because they couldn't get away with providing no optionality control, so they went for the option which pushed as many users as possible to their system.
Q When did I or Slashdot give implied consent to anyone to inspect the packets for reasons other than routing?
A You didn't, but Phorm and the spineless UK government has decided you did.
Q What data do they collect and what do they do with it?
A Browsing habits to produce targeted advertising.