I presume that the hardcoded minimum difficulty is done based on the typical hashrate seen throughout the lifetime of TRC network, i.e. around 30~40 G h/s from diehard supporters.  The inability to go lower would significantly reduce the problem of huge numbers of orphan/invalid blocks seen at really low difficulty.  Hopefully, this would enable pools like Coinotron to bring back the TRC pool and make the network more secure.