Some transactions must have a fee, and that is determined by the network, not Armory. Armory simply determines whether the network will require a fee, and then tells you you must include it. Many transactions, especially those over 1 BTC, can usually be sent for 0.0 fee.
Small transactions, using coins that were recently received, almost always requite a fee of 0.0005. The network does this to prevent people for sending out millions of tiny transactions for free, or moving coins billions of times between two of their own wallets and clogging the network.
"Moving coins billions of times between two of their own wallets and clogging the network"
can also be done with 1 or more BTC.
Small sums are normally used for tests, to check the functionality of backups, clients etc.
See no reason to punish tests with fees.
"input_age" is also considered when calculating the priority, which partially determines if a fee should be applied.
And it's ultimately not up to Armory to decide if you should pay a fee or not, if miners don't want to include your transaction for the lack of a fee included, they won't deliver you a message to inform you.