I just...don't see the offense to be taken when other people want to use the term in the way that the Japanese do.
"Offense" is too strong a word. I think it's foolish, but other people do a lot of things I think are foolish (and I
know I do some things other people would think to be foolish). Generally I'll abide by a "live and let live" policy until, as in this thread, someone decides to ask me point blank about my opinion.
Now, as for
why I think it's foolish, it boils down to two things.
1)
We aren't speaking Japanese. Because of that, there is absolutely nothing gained from taking a Japanese loanword and trying to preserve its original meaning. In fact, it makes usage of the word in English more confusing when you attempt to use it broadly, as per its original meaning. In modern English, generally words from other languages are used to indicate things from other cultures. We call Mexican tripe stew "menudo" to indicate its origin. The same goes for the dumplings called "runza" imported to the Midwest from German cuisine, and most American noodle dishes based on Italian types of pasta (spaghetti). This is largely the rationale behind calling Japanese cartoons "anime" and Japanese comics "manga," and attempts to change those definitions post hoc were met with stiff resistance. Deciding that tokusatsu is going to work differently doesn't make sense, unless you ascribe some special emotional value to use of the word. That is, it makes no sense to call not-Japanese things tokusatsu, unless doing so just happens to make you feel good. For purposes of communication, it's pretty obviously not the best choice to make, because English has more common terms available to describe the exact same things.
2)
That's not how loan words work. Precious few Japanese words that have come into broad usage in English have anything like their original meaning. Generally, the ones that do refer to very specific things that in English would be nouns, things like tsunami or sushi. But tycoon in English is a million miles away from the original meaning of taikun, and the same is true of words like honcho and other older Japanese imports. At the end of the day, words are tools used for communicating ideas. Their value is tied to their usefulness for describing the world and the things we encounter in it. Old words fall out of use in English and "die" all the time, usually because the things they described stopped being relevant. Likewise, you have words whose meanings change and mutate. Loanwords picking up meanings different than their original meanings is just part of that natural process, and I see no point in fighting it. If we do, we're stuck saying that the only legit meaning of "hamburger" is "someone from Hamburg," and I honestly don't think anyone here would seriously advocate that.
Other than coming off as though you are talking down to fans about what choices they make in what they call things, would you go out of your way to tell some one that what they call toku isn't?
It would depend on how well I knew that person. For someone I knew really well, I would have a "Uh, what? Why are you doing that?" reaction, because I would be very confused and want clarification.
For a more casual acquaintance, or typically on a public forum like this, I would smile and nod and probably go with the flow. Chances are the topic of conversation is something else entirely that I'm more interested in, so I'll focus on that instead. And while someone might use tokusatsu in a way I don't agree with, if I can figure out what they're trying to say at all, I'll continue the conversation based on that.
Generally I'll only bring up my feelings about what the word means and how it should be used in a thread like this, which is... about how the word should be used. Otherwise, I think bringing semantics into casual conversation about other topics is pretty boorish and should be avoided. Most people don't find it enjoyable and just want to talk about the thing they want to talk about.