Air Canada reserves a block of seats for crew rest during long flights. This is primarily for safety reasons I believe; rested crews perform better in emergencies based on what I've read and of course we're all entitled to breaks at work on long shifts like this. In general, I would say that if you have a medical condition that makes sitting difficult, you may want to consider business class. While this is expensive, you might be able to do something with upgrades etc to keep the cost more reasonable. It certainly was probably annoying to see that the seats weren't used, but unfortunately it's contractually something the flight attendants are entitled to. I don't really want to sound like a jerk, but it's important to remember that you should only expect what you're entitled to based on what you paid with airlines, and what's mandated by law such as mobility assistance (wheelchairs etc) and sciatica isn't a defined disability requiring accommodation by law. Orienting yourself to that understanding will make travel a lot less aggravating for you (and I suspect a lot of other people). The only way around this is to pay for the space you need to lie down. It's not nice, but it's just reality.