Do you need to register? We are going Monday to Thursday and would like to ride a lot and look around. Just interested why I need to register?
This ^ sounds like you want to go and be a part of the event without bothering about registering...
As a person who's been involved in putting more than just a couple of similar events on, but does no longer because of the personal and financial cost and commitment doing this entails (I was forced to retire cos my body rejected the amount of work, and organising/running events was no less draining!) I reckon that I might understand a little more than most the amount of often unappreciated time, $$, and effort that goes into the organisation of this sort of event, so I personally reckon that it'd still be very cheap and no true reflection of the actual cost involved at twice the registration fee.
That said, I'd suggest that if you're going there at the time of the event and want to partake &/or be a part of the event in any way; if you intend to take part in any of the rides, etc; &/or avail yourself of any of the facilities/accommodation/events/vendors etc that the organisers have arranged/put on as a part of the event, then you really should register, not only out of politeness, but also at the very least to let the organisers know that they've actually got however many people people coming and taking part so that they can ensure they've got everything they need covered for all the people they know are attending!
It's tough enough pulling something like this together to start with, but if people just rock up and expect to be able to have anything to do with the event without registering, maybe even causing some who did do the right thing and registered to miss out, then how long do you think the organisers are going to bother continuing to make the significant effort and incurring all the costs of arranging and organising an event like this just to let the free-loaders bludge off those who did do the right thing?!
If you want to be there and have anything to do with the event, then you should register and be a part of it; or you can choose not to register and instead, potentially be a part of ruining it for everyone else, not just for the organisers, but also for all those who've done the right thing and registered, and maybe even contributing to making the event cease happening.

Do you really need to ask me how I know this sort of thing
does occur?!
Just Sayin'