Yeah, the average sports bettor, just like the average casino gambler, faces a mathematical disadvantage. Unlike casino games, the outcome in sports matches is not entirely random. With that in mind, even though the oddsmakers are very good at setting point spreads, they are not always right. These facts give an advantage to experienced gamblers in sports betting.
You are correct, but it works in some special cases. For example, knowing the insider, I can better than the bookmaker understand the possible chances of teams in a particular match. But this does not matter at all at the distance, since the advantage that the bookmaker has at the distance turns into a 100% win for him.