What you are saying ref. IG is not strictly correct.
They were taking bets but not limited risk bets. You will find that most SB companies limit there exposure to limited risk bets even on larger ftse 100 companies. This sometimes has regard to the binary nature of a situation. ie something will fail or succeed.IE AZM prior to these results or Scottish and Newcastle 3 months ago after yet another buyout rumour. Sometimes it is just a matter of numerical limit.
Normal bets are not a problem as they are hedged in the market by an actual position. SB companies sometimes buy so much on behalf of the punters they take declarable positions in companies.