网友误读了这个句子。这其实是一种简略的并列结构：A，(but) not B. 肯定A，否定B。but可以省略。例如：我要把这本书给汤姆，而不是给你。英文为：I will give this book to Tom, (but) not you.

这种结构中，A可能是句子的主语，也可能是句子中的动词或介词的宾语。

Tom did it, not you. A为主语。

I like Tom, not you. A为动词宾语。

This is for Tom, not you. A 为介词宾语。

注意A和B之间既不是同位关系，也不是修饰关系，而是并列关系。即not B是第二个并列分句省略的结果，省略了相同的句子成分，只保留了不同成分。

网友的这个句子就是这种A，(but) not B 并列结构，省略了but. 其中 A=private charity；B = something people are supposed to need in a proletarian paradise。

B 是一个名词性结构，名词性结构不可能作后置定语，排除后置定语。

not B 不可能是同位语。

A 和 B 并不是修饰关系，而是并列关系。用省略which is来解释是对句子意思的严重误解。该句的正确解读，是补上省略的but:

All this may help explain the government’s surprising embrace of private charity,  but not something people are supposed to need in a proletarian paradise.

以下为COCA有关 A，（but) not B 的部分例句，既有带but的，也有省略but的。为求简便以 （but ) not you 搜索。

Every Handmaid who followed you into disobedience will face the

consequences, but not you.

Let others leave their futures in other people's hands, but not you.

All the technicians etc. should get paid, but not you. Why?

Why was I sent away, but not you?

They probably hate me, but not you. Listen, all that stuff, my fault.

I expect that from your mother, but not you.

What will make that person, not you, feel better？

My concern is for the country, not you guys.

I am not the one who worked 37 years for the federal government, not you.

He was the one who broke your trust, not you. 

You may " blame " Obama, but Congress agrees with him, not you.

Eventually, the responsibility for their issues falls on them, not you.