This is quite interesting, considering I've heard of at least 3 people with Bluefin who had their 1st and 2nd gear syncro break down.
I'd expect the limitation from Ford is there to avoid exactly that problem - the torque churned out by the gearbox is after all higher in lower gears.
On a side note the ST also limits the boost in 1st and 2nd.
Furthermore my stock Black Edition (got both a ZS and a Black Ed.) feels quicker in 3rd than in 2nd, I suppose this is also down to limited boost in 2nd gear.