Bypass Capacitors - Why do they not provide voltage spikes back down the line?
Spikes are very short events (otherwise, they would not be formed like a spike), so they do not have much energy and cannot add much charge to the capacitor, so the output voltage does not actually rise by a noticeable amount.

(If you'd connect a 6 V power supply, it would shove as much current as needed into the circuit.)

From an alternate (frequency) point of view, the capacitor has a low impedance at high frequencies, so the high-frequency spike gets shorted to ground.