Can current flow ever oppose a current source?
This circuit only works if the current through VCCS1 is minus the current through VCCS2. So the input has to be purely differential. In every other case there would be no path for the current.

In a real circuit Re would be split into two 25 ohm resistors with a large resistor (resulting from a non-ideal current source) going from the middle tap to ground. This would create a voltage that counters the common-mode component (-> differential pair).