Like a check valve? Does that mean that some (clean) water is trapped between the pump's output and the seal at the end of the shot? If so, this doesn't seem so terrible.

One thing I've often found odd about espresso machines is that they all seem to have some mechanism to depressurize the basket when a shot ends, and that this mechanism lets water that may have been in contact with the grounds go through some portion of the machine. What's the point? The pressure will naturally dissipate quite quickly through the grounds unless like kind of pressure-retaining basket is in use.