You went a pretty long way toward answering your own question here. Maybe you just needed to get it off your chest. :)
I know it's probably because they assume that since both MS and Oracle adhere to the SQL standard (loosely), that it's all the same.
That's probably at least a small part of it. If you've only ever used one or the other you might not know how big the difference is.
And in some cases, even the wrong syntax will help someone who's just looking for the best approach to the problem - if they're strong enough in Oracle they'll be able to convert the syntax.
This is probably the bigger reason. I think most people who use one or the other are probably aware that there are differences. As you point out though, sometimes the general approach can be helpful. I think the main point is that even if your answer isn't directly helpful to the person who originally posted the question, it might later be helpful to someone else with the same problem on a different database.
Should I keep downvoting these answers when they are obviously unhelpful?
It's up to you, but I don't downvote an answer unless it's actively unhelpful (misleading, malicious, doesn't answer the question, or is in some other way just totally wrong). If an answer is just not helpful (subtle difference, I know) I'll just not upvote it.