More than any other European leader, and largely unnoticed in the UK, Merkel has moved Germany from a wholly compliant NATO member to an equivocal bridge between Europe and Russia. Sometimes she swings behind Putin, denying German air freighters, tarmac and logistics to Britain and the US. Sometimes (as with Afghanistan) she swings Germany behind her NATO allies. She wants to both run with the hare and hunt with the hounds.
And as long as Putin's and Russia's communications are legitimate to intercept then so must be all of his allies, however temporary. That includes Merkel. Obama may have declared clearly that the US no longer monitors Merkel's phone traffic - but he didn't make the same declaration for GCHQ.
For once, Boy Cameron should just keep his mouth shut and refuse to comment. Knowing what's going on in Berlin and Paris as well as Brussels is vital to the UK's interests.