I think the rules have got stricter and stricter. Not so long ago players were free to bet on other games.
I used to work next door to a bookies in Cardiff about 15 years ago and would put a few quid on the dogs at lunchtime once or twice a week. I regularly saw loads of city players in there betting hundreds and thousands. I don't know if things have changed but it seemed be be almost an embedded cultural behaviour in footballers. The amount of spare time they get can't help either I guess