Their rules,moan the banks,have forced them to report enormous losses,and it’s just not fair.

这句话可以改写为：The banks complain that their rules have forced them to report enormous losses,and it’s just not fair.这就容易理解了.
正确的译文是：那些银行抱怨说,它们的规则迫使它们报告遭受巨大损失,而简直这是不公平的.
“moan the banks”绝对不是同位语,而是插入语.如果把它抽掉,并不影响原句结果,反而使你更容易理解句子了.今后可用此办法来对待这种句子.
另外,“moan the banks”是将主谓语倒装了,反而加重了语气.当然,也可以不倒过来,而说...,the banks moan (=complain),...
最后请注意,插入语前后都有一逗号.
望以上介绍对你有所帮助.