I would have to agree that sports are probably even better source of income than casinos, but the reasons why books might feel the need to scam affiliates are obvious. Books carry a lot more risk than casinos as sports betting ain´t just about luck as there are so many variables and I know a lot of guyes that make a living from sportsbetting(you dont want those guyes on your revshare 
) …unless that book would be a betting exchange, but then again I think there is not one exchange that let´s you keep the player for life – and so that takes the wind out of exchanges.(Please let me know if you have found one.)
For example even I have been a succesfull sports bettor time to time and I still do bet on big events and F1 races (I am roughly 2,5k up for the F1 for this year and 0,5k on the current Euro2004), but in order to run a true betting site I think one must do a hell of a lot more work than with casinos and still you might end up at nothing as some talented bettors can take away your hard earned money and this is why I am doing casinos over sports. If I knew where and how to attract NEW bettors that are not so called professionals then I would surely jump on it, I dont know what is the basic type of north american sportsbettor, but the ones I know from here are quite talented and use their money carefully and in the same time use up to 10 different bookies in order to get the best price and some times even so called sure things where you win what ever the result might be. The more desireble “stupid” bettors are not using so much web as they tend to play with the local landbased bettingshops that we have on every corner 
I think that the sportsbettors are a community and they prefer to exchange their thoughts on discussionboards and from there they find out the best places to bet so basicly it´s working like MLM-marketing and they dont even need to pay for it. Now if you are thinking – Ok, I will now start a board that is about betting, well I must say good luck and enjoy ghost writing for few months as I think a discussionboard needs a lot of people to really function and also quality material to enjoy. I was about to start my own betting site for the Finns and so I did some research on the other non-profitable boards to see if there´s interest and as you can guess the welcoming of a new “commercial” site was not very friendly.
Now if I would be starting a sportsbetting site I dont really know what new I could bring to the world of sportsbetting either than a livescore service or create a themed site where I would post my personal pick´s of the day and I think there´s a lot of these around as well. One site that I find appealing is GoonersGuide.com where the owner of the site is a big soccer fan and supports his team, clearly a gooner(Arsenal Fan). I like his reviews and the way that he analyses the games although I think the site would need a lot of work in means of load up times and it is not clearly a betting tips site more just a overlook on the games in hand.
Well before I stop writing this non-sense I must say there is a quite a tricky issue involving the commerciality and good betting tips as if you are hitting your marks and teaching people the right way of sports betting you are soon making nothing as the players stop loosing . So why would you then do a lot of work to find the best odds and profitable marks as that would mean that you loose money when you hit your targets Doesn´t make sense or does it?
My own summary would be that in order to make money with sports you have to find stupid freshmans or get paid for placement or the most obvious – Get on the BettingExchanges as then you will be making money on good tips and good players – Just like poker!