garden flocks

While I watched a sparrowhawk being mobbed by martins today as it hovered over the garden looking for sparrows, and a humming bird moth on the garden phlox something else caught my eye. Large bumblebees kept on disappearing into the heads of garden phlox (see top photo). On closer inspection it transpired that the bees were feeding from the base of the flower, between sepal and petal tube (see lower photo) rather than from the flower heads themselves. Is this common? I haven't found a reference to this behaviour on some brief internet searches. It would seem to negate the normal method of pollination.

PS I have found a reference now to this behaviour, it is called nectar robbing (see link). The reason it did not come up at first is that this site does not mention phlox. Looking at the flower one can see it has a very long petal tube, and I guess these bumblebees cannot reach the nectar at the bottom, so have to drill a hole in the petal tube... fascinating.