It is because I think that "antagonize", "oppose" and "obstruct" are actions against people.
Maybe but, as Tdol said, "An example that does not require a preposition is not the best way to test whether a preposition can be introduced."
If you have further questions, please ask them,
one a a time. Wait until you are fully satisfied with the answers to one question before asking another one.
Do bear in mind that issues like these are often about style as much as about grammar. You should not be surprised if even educated native speakers do not always agree. This does not mean that one is wrong, simply that there may be no clear answer.