If someone was infected with crabs and they sit on the toilet seat then yes the crabs could be transferred to the toilet seat and then when you sit down on the toilet seat the crabs could then jump onto you and attach to your pubic area.
So it's a good idea to put down some toilet paper on a public toilet seat or not even sit down on the toilet seat if possible.
I hate using public toilets but sometimes have too and luckily I haven't gotten crabs yet from the toilet seat.