The fact they have different companies posting the same or similar jobs is not a rip off, provided the jobs are real. Each of these is not only setup with different dbases, but they are also probably setup as different companies. This way, they can reach out to people using different search criteria. Lots of search firms do this and they are all setup in the same way - a different company requires a seperate profile. So the answer to your question, "if I have an account with employment crossing, shouldn't I be able to apply to lawcrossing with my employment crossing account?" is 100% NO for all of the reasons I just explained.
If you REALLY want a job, ask yourself this: Do you want a job, or do you want to complain about entering your profile in different places? Chances are, you want a job. It may not be the most efficient in your eyes, but it is that way for a reason.
In the end, it isn't customer service's responsibility to prove anything to you. If you want a job and want to use the service, that's your decision. If you want a job and wish to use another service, you can do that too. Choice, not a rip off.