1. You get to test the flaws in an environment where nobody will raise an eyebrow. If you go straight for the production system, it is likely your early attempts will visibly show up in the logs.
2. You get paid to carry out malicious deeds. That's a double win.
It would be kind of silly not to.
The second two sentences read like excellent reasons why you should take the job (even if they are just a repeat what I already said in different words).
I must have missed something.
You get well paid and it's legal.
The reality is that you don't get paid well if the data is worthless. You only get paid well when the data is worth orders of magnitude more than what you're being offered. If you are inclined to break that law, that's a pretty nice carrot dangling there.
If you are so inclined, why wouldn't you take the job and report the not so crafty exploits to bring in the sweet, sweet paycheque and use the really juicy exploit to also go after the even sweeter data? It's a total win-win situation...
...unless you get caught, but if you are so inclined that's not exactly on your radar.
[1] Okay, regulation, but the need for such regulation is still in question.