When a TWIST ID is registered, a transaction is debited from the registering address for the amount of 50 TWIST (currently around $0.15). This 50 TWIST fee is a necessary control to deter individuals from registering large amounts of TWIST IDs.
That is an interesting concept.
If we had a small fee (e.g. 1 TWIST), an individual could very affordably register thousands of TWIST ID's (e.g. using common first names or popular usernames). Those ID's would then be unavailable to register for other users.