The vast majority of *hardcore* fans, the ones who eat the high costs of cable/satellite packages because they want to see their favorite team on their RSNs, do want to see these games. As far as advertisers, don't they pay up for live sports programming (as opposed to the 3rd rerun* of the day for the Dan Patrick show), because they know viewers can't zap their imbecilic commercials?
But hardcore fans don't make the world go around. The casual fans do. If teams had to rely on the hardcore fans, they'd be out of business in a month. Secondly, the monthly subscription the network gets for the hardcore fans isn't enough to fund the network. They have to have advertising dollars. Lots of them. And yes advertisers pay for live sports programming, but not for live practice.
Professional sports are not like college. Yes, everytime the Longhorn band has a bake sale, there are hundreds of thousands of screaming fans ready to pony up hundreds of dollars to watch a piccolo player hand out change. But the viewing public simply does not care about the Astros' practice games. Bottom line. And it's all about the bottom line.
*And in an age of DVRs, why the fuck re-run programming at all?
Because it's cheap. That's the point.