Hey, there! I'm from Adelaide, but I'm pretty sure you can still catch one em recent episodes on Nine. But I just go for torrent anyway.
Hmmm. Well, you do have a fairly good point. I think it's because Sheldon has been fairly patient at the start of the episodes, and since Penny (a complete stranger) and he might've been trying to act 'polite' (it's possible). Haha. I think he didn't expect Penny to be hanging out with them most of the time, maybe that's why he wasn't as up-tight as he is now.
I remember in one of the episodes, he had given Penny 'strikes' and tried to use the 'Roommate Agreement' on her but at a certain point, some of Penny's privileges were stripped off and she tattle-taled to Shelly's mum. Since then he currently can't use the strikes or Penny'll tattle-tale to his mum. So I'm assuming that whatever chance he gets, he'd bring out the Roommate Agreement. I don't know if I'm making sense, but yeah.
And about his patience, sometimes, Sheldon does give in, there was one episode when Sheldon dressed up as the Flash and Leonard asked him if he could turn the thermostat down (even though there is a constant temperature in the Roommate Agreement) and then the Flash-Sheldon, annoyed by Leonard, 'ran' to the Grand Canyon, screamed his lungs out, ran back and then calmly okayed Leonard.
Just expressing my theories!