My understanding is that US banks can write a proportion of the loss off against tax, so 50/50 splits are not that unusual. But it probably depends on the circumstances.
I'm in the UK and this is not really my area, so you'd probably have a better chance of verifying that than me.
We actually had Amex _negotiate_ with us over a chargeback once. I had no idea this was even possible -- they actually said "So how about we'll agree to split the loss. We'll cover half, you guys cover half." I had no idea that was even an option.