Because economics change in 8 years, that you yourself also very obviously know. Within the last 12 months, 3, THREE, superhero films have crossed a billion.
Surely its thunderstruck entire Hollywood, which is a business afterall, at the financial gold mine these films are.
Which or how many of these 3 films were reboots/start ups to very unsuccessful crap? Within the last 12 month that is? Studios expect that sort of success from tested properties/investments no matter what the new thing is.
And also the investment in MOS was much higher than is normally for first films. Just look at this - the first film of a solo film series with no stars cost more than the group film of Marvel made after 4 solo films and with an all star cast.
It's good business on marvels part. They have been criticized and celebrated for this very thing. How do they get around having an all star cast? Google any of the various articles about the actor salary controversy at marvel over the past year. Moreover, I haven't had a chance to look at RDJ's contract but I doubt he get's paid 50mill up front.
It's a different business model, I'm sure like most of WB's projects, JLA will have an inflated budget.
Looking at the sheer investment, WB would be within the rights to expect more. I think everybody did. 600 something worldwide isn't bad but then you think that Avengers made that in the US and you start thinking wow.
This last post suggests to me that you have a mind for circumstances, if you apply them to the superman property and the Ironman one as of this year I'm sure you can rationalize why the studios no doubt had different expectations for these two films this summer(go see IM two's numbers for a more accurate comparison).
Green Lantern costs more than MOS as well and I'm pretty sure WB didn't expect that film to hit a billion. They expected a property starting success, it's good business. ASM costing more than MOS and making the same and still being considered a resounding studio satisfying relaunch speaks volumes on the matter and the state of things in this economy.
Moreover the cost to realize the scale in this superman movie was no doubt the same as it was to realize the scale of that avengers film. Expectations aside, studios also tend to assign concept realizing budgets(between MoS and Returns we have an idea of how much it costs to produce a high concept superman film).
For your logic to be sound that would suggest WB should have spent on MOS half the budget Marvel spent on Avengers (42% to be precise), We are talking revenue here not profit. That would mean for the WB to come out successful in their investment we are talking about greeen lighting a superman reboot with the estimated budget of around 94.2 million.

My math sucks btw.