The idea behind Bookcrossing is not to steal royalties from authors; but to promote reading!!
It's like the whole controversy now concerning music file sharing on the internet. I heard someone on the CBC relating the laws of file sharing to having a photocopying machine in a library, and I think that's 100% valid. In a library,(any given library),you're surrounded by copyrighted material and there's a photocopy machine in the corner willing and waiting for you to copy pages, either to share for research or pleasure. The same works with internet music sharing. You have something you like and you make copies of it and pass it around.
I remember the days before c.d's, where i'd wait with an audio cassette ready in my ghetto blaster, tuned to a radio station, for my favorite song to be played; press play and record, and make my own mixed tapes of radio stuff for my friends in high school. No one was complaining then...
Anyway, to get your shackles up because a bunch of people are sharing your book and probably enjoying it, without you, the author, making money off of the sales is ridiculous.
Bookcrossing is a godsend, especially for book lovers, such as myself. I just heard about it, signed up right away and sent two books into the wild not two hours ago. Books that I've loved and have multiple copies of. I believe in spreading and sharing the goodness of a book, but god forbid if i'm gonna get rid of a favorite book without securing, at least, one for myself. I'm sure there are thousands of bookcrossers like me out there 'in the wild'. There's no need to worry.
I wish I'd heard about Bookcrossing years ago, when I was travelling...i would have had quite the track record.