That's because
PR misled you. Not when they said that lots of lawyers go into IB. That is absolutely true.
PR misled you when it said that you need a strong foundation in finance and accounting. Don't get me wrong. Such a foundation would be useful. But the word 'need' is too strong.
Let me give you some examples. What majors do IB's tend to hire the most of for their analyst positions? Finance, accounting, and bus-ad majors, obviously. To some extent, econ majors. But what else? Would you be surprised to discover that IB's hire lots and lots of people who majored in engineering, CS, physics, and math? I recall that out of the MIT School of Engineering (not the Sloan School of Management, but specifically the School of Engineering), IB was and is one of the most popular employers of MIT engineers. The same can be said for other elite engineering programs. But why? Think about it. How many engineers have ever studied finance or accounting?
Now you might be thinking, well, maybe the engineers didn't study finance/accounting specifically, but engineers are very quantitative anyway, so it wouldn't take them much time to pick that stuff up. Ok, let me give you another example. Other than Wharton, what are the 4 most popular undergraduate programsthat IB's recruit at? I think it's safe to say that it's probably HYPS. But why? Specifically, if you need to have done finance/accounting to get into IB, then why are the IB's recruiting so heavily at HYPS? How many HYPS students have ever studied finance or accounting? I think it's safe to say not many. I don't even think HYPS even has an undergraduate accounting class. And as far as finance is concerned, the best you might get as an undergrad at HYPS is some hybrid version of finance that comes with certain economics undergrad classes. But certainly not any truly formal finance curriculum.
The point is that IB's are going around recruiting lots of people without backgrounds in accounting, or finance, or both. And the reason they can do that is simple. IB's are simply going to take whoever they hire and train them anyway. Is it useful for a candidate to already know some accounting and some finance so that he can get a running start? Of course. Might that candidate's background in accounting/finance then be a boost to getting that person hired by the IB? Sure. But do you absolutely need to know accounting/finance to get in? No you do not. IB's are more interested in a very strong work ethic. That's why they hire lots of MIT engineers, because if anybody has proved that they have the self-discipline and drive to work long and grueling hours, it's the MIT engineers. They also hire pure raw talent, which is why they recruit at the very very top schools, where you obviously have to be highly talented to have even gotten admitted in the first place (in this sense, they are outsourcing part of their HR department to the school's adcom). They are also hiring a 'network'. Banking is really about making deals, and so the people that you know from school can become very important. Graduating from a big-name school gives you access to a tremendously strong alumni network which can and often does come in extremely handy when you're trying to complete a deal. And of course, banks are hiring 'prestige'. Banking is in many ways a sales job, and it's important for banks to present themselves well to clients. A bank will obviously look better when it can say "The guys we're assigning to your account are all Harvard graduates."
Hence, I think you can now understand why IB's hire law-school graduates. First off, it's obviously not all law-school graduates, but predominantly those who graduate from name-brand law schools like HYS. Secondly, it's much of the same reason that IB's hire engineers - they want to bring in guys who work hard and have raw talent, and if you can get through law school, then you probably worked hard and have raw talent.