The free TX policy has a coin days destroyed. For example, sending 100 BTC around would allow you to get free TX 100x faster than sending 1 BTC around.
Aha - I thought something like that might be the reason - so does the figure of 300 BTC have any "magical property" in regards that it seemingly can get into every new block (despite the UTXO being no older than the last block)?
If they used 100 BTC instead would that not work just as well?