1080i TV plays in 720p

[jeng]

My TV is capable of playing 1080i and many HD television programs do come in in 1080i. Upon setting up my Blu-Ray player, it's playing everything in 720p. I go into the setup menu of the player and the option to set it to 1080i is darkened and i cannot set it to that. I am connected via an HDMI cable. I understand that there may not be much difference, and perhaps 720p is even better, but why can't i set it to 1080i?