同位语,顾名思意,就是从句与先行词是在句子中是一样的成分.也就是说,去掉同位语从句句子在语法上还是成立的.
而表语从句不同.表语是一句句子,去掉了表语从句句子语法上语意上都会出现问题.
你举得例子的问题在于：The news came that we had won the first prize中that we had won the first prize确实是news的同位语.但是这不是一句句子.这只是先行词+同位语从句而已
而The news was that we had won the first prize这是一句完整的句子了已经.
The news that the puma has been at large now is widely spreaded
这句话中才是一个完整的句子.主干是The news now is widely spreaded
that the puma has been at large是同位语从句,补充说明news的内容.去掉的话句子还是成立的