"I Am Legend" was produced in 2007 but Warner Brothers had already heavily trailed the possibility of a Batman/Superman crossover film as early as 2004, going as far as confirming a Director and Head of Studio (neither of whom ultimately ended up working on the project).
The logo used in both "I Am Legend" and the upcoming "Batman Vs Superman" film is a variant of the Superman/Batman logo used in the crossover comic of the same name back in 2003.
It's also worth mentioning that the Producer of I Am Legend (Akiva Goldsman) was also involved in rewriting an aborted Superman/Batman film screenplay as far back as 2002. You can read the failed screenplay here. Putting the logo in the film may have been a subtle jibe at Warner Bros.
Director Francis Lawrence has confirm this info in a recent interview with Collider. It was indeed down to Akiva Goldsman that the logo was chosen (hat tip to @collinvanuden) for the link.