http://www.thefa.com/football-rules-governance/laws/football-11-11/law-12---fouls-and-misconduct
Given Targett slipped and then Fabregas fell over his outstretched leg, it is debatable that his challenge was "careless, reckless or using excessive force" ergo the referee's interpretation wasn't unreasonable IMO.
I guess it could be argued that Targett "tripped an opponent" by being "careless", but given he may not have made contact if he hadn't slipped then that's also debatable. It's possible to have an accident without being "careless".