Many people who use nero are users who either used windows previously or are dualbooting between windows and linux. and consider the fact that most OEMs bundel Nero for windows with thier products.
I suppose the forced registeration for Nero 6 serves two main purposes:
1) Users who own Nero for windows (many people do) and OEM users get to have nerolinux for no additional charges or a little fee for OEM users.
2) Nero Inc. gets to avoid building a new process for payment, marketing, managing users and serials, etc etc. Also they get away with no formal support for the product since it's provided as a free value-added product for thier customers. New users or users who won't use windows have to pay alot to get so little compared to windows users (there is no full nero suit like there is for windows. etc) but I think that's not a problem they're considering right now ~_~.
DISCALIMER: I do not work for Nero Inc. nor do I have any relation with Nero Inc. execept being a customer.