You always have interesting questions, Oscar. There are many valid choices for the prepositions. They have slightly different meanings: "What is your basis for disagreeing FOR/WITH/ABOUT this proposal?" FOR emphasizes the choice of proposal. Why are you disagreeing about this proposal, as opposed to your reasons for disagreeing with some other proposals? In this use, the word "for" is attached to "this": "for this". WITH emphasizes disagreement with that particular proposal. In most cases, this is your simplest and best choice. ABOUT emphasizes the content of the proposal. What about the content of this proposal makes you disagree with it? Why does this proposal "smell" bad to you?

The example is incorrect for North American English [I can't comment on other English versions]. Books are usually professionally edited, and the English in them is standard for their country/market. Internet articles are rarely edited and the English varies from excellent to incorrect. Some incorrect phrases are due to sloppy work, such as changing a phrase multiple times and not noticing a typing mistkae [<<] or a cut-and-paste error error [<<]. "For" and "because" can give a reason. Examples: "I disagree with the proposal for the reason that it is too expensive." "I disagree for the reason that it is too expensive." "I disagree because it is too expensive." "I disagree because of the cost."

The most common preposition to use with disagree is “with”. I disagree with him. You do hear “disagree on/ about”occasionally. We disagree on privatization. My partner and I disagree about climate change. “Disagree for” doesn’t sound right to me. I would avoid using it.

This Google Ngram frequency chart shows that "disagree with" is used most of the time [70% ?]. "Disagree about" and "disagree on" are less common [15% ? and 15% ?]. "disagree for" seems to be not used enough to register. https://books.google.com/ngrams/graph?content=disagree+with%2C+disagree+about%2C+disagree+on%2C+disagree+for&year_start=1800&year_end=2019&corpus=en-2019&smoothing=3