The L1 circuit is slightly different for white, red, green, and blue. Each Luxeon takes a slightly different current/voltage. The circuitry is in the body of the light and switching heads would drive the LED in the new head at a less than optimum level. I don't think that's why they don't sell the heads as replacements though.
As for the A2, I don't wonder why they don't sell additional LED rings. They don't want inexperienced people pulling their lights apart and tearing out the innards of their light, trashing them, then sending them back complaining that they don't work.
Those are mostly speculation on my part, but it seems to make sense.
