Friend, I also read a similar thread about coinbase. I don't know whats happening today. If the transaction fees is zero, then its good for the bitcoin community. But when I send some money with my electrum wallet today,transaction fees was around 0.0005Btc. Anyways you guys are lucky to have zero transaction fees.
Anyone is able to send bitcoin with zero fees. It's just that you may never get a confirmation or you may be waiting for a long time before you get a confirmation on your transaction, that is all. You are free to choose the amount of fees you pay, but wallet services usually decide for you the recommended amount.
@OP, On btc.com i think you get some free uses on their accelerator, which basically means that you are able to send your transaction with a small amount of fee and still get it confirmed in a relatively short period of time as soon as their pool mines a block. I think this is what happened here.
See https://bitcoinmagazine.com/articles/btccom-wallet-tries-discounting-some-priority-transaction-fees/ for more info.
