I was flying from PHL-ISP today and tried to standby for an earlier flight. Although there were 2 empty seats, they ended up not letting me on the flight (and they actually pulled someone off the flight) due to "weight restrictions." Is there someone who can explain what the deal is with this? Why would a plane leave with 3 empty seats? If it can't accomodate that much weight, why have the seats there? I'm sure that I am probably missing some important details, and if there's someone who can bring me up to speed on how this works, that would be great!

Also, they apparently pulled off some bags as well. In this type of situation, do they take steps to ensure that the bags they pull off are checked bags and not carry-ons (this was a Dash-8)? If they simply randomly pull off bags, people could end up at their destination with nothing but the clothes on their back, so to speak.