Whenever a new coin hits an exchange, it's almost always first traded against the Bitcoin. This was the case with Feathercoin. At this point in time, I do not know of any exchanges that are trading Feathercoins directly for USD. The Feathercoin price website that you linked is simply calculating the price of Feathercoins in USD based on the FTC -> BTC -> USD rates. This is the same thing that dustcoin does when you use the calculator (it can calculate hourly/daily/etc profits in USD). All of the coins listed at dustcoin are at minimum tradable directly for Bitcoins and thus comparing them based on this rate is the easiest method possible.
As a side-note, some alt-coins like Litecoin are traded on sites like BTC-e directly for USD. There may at times be small disparities in the value of LTC/USD vs LTC/BTC/USD but it never really becomes considerable as trading bots are quick to gobble up arbitrage opportunities.
Ultimately, when considering the profitability of any coin, I look at the rate at which it can be mined and its exchange rate against Bitcoins. This is exactly what dustcoin does for you and I highly recommend considering its advice.
