Basically, below 50' radar altitude the automation is configuring itself for the flare and roll out.
Flare begins at 50'. FMA changes from G/S to FLARE.
Between 30-15' Thrust goes to idle. FMA changes from SPD to IDLE
At 5' TOGA is disabled and FMA LOC changes to ROLLOUT
At touchdown FMA FLARE goes blank and nose is lowered to runway, spoilers should deploy and once the thrust reversers are deployed you're essentially committed to the landing.
The above is mostly a summary from a company training document.
I think the design essentially puts the burden on the pilot to MANUALLY begin the go around procedure at such a LATE POINT in the landing maneuver. The automation is busy preparing for the roll out portion and while it will not block the manual advancement of the power levers, neither will it be done automatically--remember below 5' it's transitioning to ROLLOUT with IDLE thrust.
Hope that clarifies it a little.