Citigroup didn't buy the highway operator - it was Citi Infrastructure Partners. This is a fund that is probably comprised of high net worth individuals, pensions, and other large investors from all over the world. This is also entirely separate from Citi as a bank; if Citigroup were to be deemed insolvent, and have its assets sold, Citi Infrastructure Partners would simply detach itself from the parent (this is currently happening with Lehman Brothers' various funds - if anyone wants a VC fund, it's currently up for grabs).
Where'd you read this info, by the way? I'd be interested to know if there's a news source that provided you with this incorrect analysis. There are a lot of reasons to hate these banks - and Citigroup is one of the worst of them - but that isn't the case here.