The weight of the chain ‘fall’ itself is sufficient to keep it feeding as the wheel is released (or the handle switched to un-ratchet).
Keep in mind that the closer the shoes ride to the wheels, the less air is required to apply them. The automatic slack adjusters will try to keep the foundation jointing ‘tight’ so there is minimal free play in the rigging itself. Note that the brake beams are designed to ‘float’ slightly free of the wheels (clasp brake hangers also have a ‘center of mass’ that allows them to fall slightly away from the tread). In a relatively slack linkage with multiple pin joints and pull rods it would be difficult to provide a positive spring action to push the shoes/beams away, on the same principle as your ‘it’s hard to push on a chain’.
As another consideration: many classical drum-brake systems are set up to provide ‘servo action’ when applied: they are hinged so normal rotation of the drum forces the shoes against pivots to increase leverage when the braking friction applies. It would not be wise to mirror this with railroad shoe brakes, as anything that happened to swing the brake beam into contact with the treads might cause self-application.