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I'm an international student thinking about going to law school. It seems that Biglaw is the only option after graduation as others won't sponsor H1B Visa.
So I'm wondering what are the exit opportunities for foreigners after a few years at Biglaw?
Do in-house counsel positions require citizenship?
On some V10 firm websites, I found a few Chinese JD in NYC office and a lot in HK office.
Like
http://www.skadden.com/professionals/will-h-cai
http://www.skadden.com/professionals/jing-lin
It appears that lots of them have stayed there for more than 6 six years, and are still Nth-year associates. Why do they work brutal hours and stay there for long run? No good exit options for foreigners?

Your immigration options depend on what country you are coming from. You should speak to your own immigration counsel but generally, if you're coming from Canada, you don't even have to worry because a TN is so cheap and easy, but if you are coming from China, India, or the Philippines, you should really consider whether you even want to work in the US because it would take so incredibly long to get an employment based green card (which is the long term solution).

Biglaw is not the only employer who will sponsor an H1B, but it's expensive for a small employer. You won't be able to work for the government or take advantage of any loan forgiveness programs, and many public interest employers won't have the resources to pay for an H1B. In-house probably will have the resources and at a decent sized company almost certainly already has gone through the H1B process before but the chances of getting one of those jobs straight from law school is slim to none.

Remember that an H1B is only a temporary visa, even though it can be renewed. If you leave your job and don't port to a new job (this is essentially an entirely new H1B application process with a few differences), you need to leave the US. If you don't get an H1B because of the cap, you need to leave the US. If you're from a country that's currently backed up, you could be waiting over a decade before you could even file to get a green card. So seriously think about paying the $200,000 it costs to get an American JD and what value you will get from that.