Sony is totally to blame for this.
TVs that support 1080i and not 720p are old - and many of them ARE Sony's. Sony has no excuse not to support them.
The issue just gets more funny/sad with the opposite problem with bluray movies: there is no way to watch them in 720p with a PS3. 1080p, 1080i, or 480p only. So if you have any kind of DLP or LCD screen that is 720p native, you're forced to watch Bluray in some non native res. Ok for tv's with a decent scaler, crap for tv's without.