The reason I personally, albeit only on occasion, answer JavaScript-tagged questions with a jQuery solution is a simple mistake, every now and then I see a question that would be easy (-ish) to solve with jQuery and, for some reason, don't notice that the question didn't feature the jQuery tag.
It is, on this type of occasion, important to remember Hanlon's razor:
Never attribute to malice that which is adequately explained by stupidity.
To answer the implied question in your question's title, which I choose to interpret as 'are JavaScript and jQuery tag-synonyms for each other?' the answer is 'no,' a tag of jQuery implies that a JavaScript solution would be acceptable, but a tag of JavaScript does not, in any way, imply that a jQuery solution would be acceptable or useful.