Why is 于 needed after 不屑 (but sometimes not)? In the following sentence:


  也有一些“小资女人”的领军人物开始不屑于说英文了，偶尔来句日文或法文才能彰显自己的不同。


This sentence uses 不屑于, not 不屑, but the meaning would be something like:


  Also, some leaders of “小资女人” started to look down on speaking English, and they sometimes think that to speak Japanese or French makes it clear that they are different from others. 


The translation might be a bit awkward so feel free to edit it.

I feel that the 于 here takes an object (说英文) after that. But in other usages, 不屑 just takes an object directly, without any preposition.

Or if you relate it to English, it looks like 不屑 here works as an intransitive verb, and 于 is a preposition. But in other uses I have seen, it works as if a transitive verb.

However, why does 不屑 need to take 于 here, even though in other cases it does not need?

note:https://baike.baidu.com/item/%E4%B8%8D%E5%B1%91/3178
不屑于 is a set phrase.
Check out《现代汉语规范词典》's distinctions:

不屑
1 动 不值得
不屑一切 | 不屑与之争辩
2 形 形容轻视的样子
一副不屑的神情。

and

不屑于
动 认为不值得做或不值得理睬
他很自负, 好像不屑于做这种具体工作。

《规范》defines the two slightly differently while treating 不屑于 as a separate independent word and not a construct.