Why play with everyone else if you can get everyone to buy your standard?
When your machine works just the same as everyone elses' then people can choose which company to buy from and might not choose yours. If iTunes was opened up to all the other MP3 players out there iPod sales might drop—of course, they might not as well—so that means it's not a good strategy. Same thing applies with BluRay and HDDVD.
Though at least Sony and Toshiba learned from the past—ie Beta v. VHS.
Sony did not licence Beta out hoping to get the market all to themselves. JVC, however, licensed VHS tech to anyone who wanted it and that combined with the longer playing time of VHS tapes lead VHS to the win.
Toshiba and Sony have virtually identical products and they have many licensees for them as well, so this competition will truly be judged by the consumers unlike the last which really only had one outcome regardless.