The fact of the matter is that Bush waited until after Obama had won the 2008 election to sign that agreement -- for whatever that's worth. If what you've said is true, then why would Bush negotiate an agreement at all?
I said they were "assumptions. And I granted that "nobody knew for sure".
However, it's a fairly safe assumption that Obama would be looking to throw a monkey wrench in the works and seeking an opportunity for an early exit from Iraq.
Why did Bush sign the deal? Because, after Obama's meddling, it was the best he could get.
There are plenty of other reasons to blame Bush. But the status of forces agreement with Iraq probably isn't one of them.