Optus wants people to have to sign up to their mobile/internet services to be able to watch PL, that's always been their goal. Allowing it to be broadcast on Foxtel negates that, hence they won't do it.
It's all about increasing their subscription numbers rather than just "owning" the rights. Whether it's working or not is another story (figures suggest it currently isn't), but the current system means that any current non-optus customers need to switch over to them for the access to the PL and then more often than not pay an additional fee for the PL as well. It's also an incentive for current Optus customers to stay with them rather than switch over to Telstra or another provider. I don't think they really care how many people are actually watching the PL (especially because 90% of the ads are for themselves!), as long as it improves their number of mobile/internet subscriptions and reduces the number of people leaving.