Yes, the game must be pre-ordered if you want to have your say in the final product.
We do pay testers for internal testing but we also want feedback from people who are 'real' players so we don't miss the mark by too much.
We don't do open beta tests because there are just too many people to interact with and we wouldn't be able to control the signal to noise ratio when it came to useful feedback. Realistically, most open beta tests are marketing ploys that appear fairly close to release and are not designed to find anything more than minor bugs and they are released sufficiently close to release that no real changes could actually be implemented. In such a case, the maximum number of people is desireable. Sins was released as a beta almost a year before release so we've left a lot of time for people to have their say in making this the best game possible.
Some companies do have limited open betas (certain number of keys) however in this case there is no guarantee we are getting people who are actually very enthusiastic about the game and will give us helpful feedback. It also has the potential to exclude and frustrate players who are so excited about the game that they are willing to pay up to a year in advance - those are the people we want to be chatting with on a day to day basis.
In summary, by charging for the pre-order we A.) guarantee that our testers have a vested interest in making Sins good and B.) keep the numbers down, and C.) give our fans a chance to have an influence and enjoy the game before release (even in beta it is quite stable and fun).
A year long beta process is a rare event and the experience alone maybe worth the price of admission - and you get to go home with a great game when the adventure is done 