You're shooting the messenger here. Their ability to raise capital has been hurt by their legal liabilities, not by the dividend cut. In fact the dividend cut helps their ability to raise capital, not hurts it!

What this move does is transfer money from BP's investors to inevitable creditors. This therefore costs investors money up front, and reduces the chances of eventual bankruptcy. That makes it harder for them to raise money by selling stock, and easier to raise it in the credit markets. With their current problems they are better off going to the credit markets, so this helps them.

And who says 20 billion is the limit? A lot of people fear it's just the start.

I agree that 20 billion is a lower limit of their likely liabilities. Which is why they should start saving up to pay those liabilities as soon as possible.