And/or make sure people need to pay attention to his spells to see which of his bullets are fake. We had this question asked specifically in the context of Danmaku games in my topic to ask for spellcards of people's OCs, Which you answered to with Oscar Diggs' character in what was then still Illusionary Nirvana's iteration of him.
I believe part of the reason for that is writing convention. What interesting story can you make with a normal (and thus not important enough to mention) human that would not include the question of Youkaihood? (Disregarding Reimu's ruthless extermination of villagers turned Youkai, that is.) It is much easier for the average Touhou fan, who know more about the Youkai part of the Gensokyo equation than the human villager one, to empathize with the Youkai. Though I feel like they've overdone it... What's also noted down is that the Villagers are meant as sheep to fear the Youkai to exist out of their belief: so giving them a vehicle to become powerful enough to not fear Youkai is... well, you get the point.
It is this strange dichotomy - Youkai in the human village, yet forbidden from entering in the first place - that gives us a conflict that is being used by everything that even touches upon the village. Most people simply don't read that far into the situation to understand the concept and the reason why ZUN made that decision.
Also, what counts as a true human to you? One who can't use magic; or one who uses magic that does not require turning into a different being? Because one actually can't partake in battle and the other has to use magic that barely exists. Even Christian witchcraft can be argued to slowly turn you into a demon or an angel depending on what flavour your magic is.
And then the part actually directed towards me; I truly do enjoy putting my long nose into other people's business.
Well, you could've at least spent a line making it clear that he shares inventory, then this misunderstanding would not have happened. (But who am I to talk?)
I assume you haven't heard of the spellcard: 'Gun sign: "3D Printer Gun"' then?
I don't exactly care whether or not a person seems like a good gal or not, and considering the amount of knowledge Oscar had about her he would have gone along a similar line of reasoning as what I stated. Both my and Oscar's line of reasoning mixed into each other, which led to that very suspicious attitude I wrote in. As for her other methods of being an omen of the destruction of his less than ordinary but still cozy life, she isn't in direct contact with anyone threatening aside from Reimu. Given time, she would be able to utterly crush any chance of a normal life Oscar has, but that time won't exactly be given without a good reason, and anything concerning the Hakurei miko will involve useful explanations to weaponize later.
Oscar is an anomaly, which is enough to get him on the bad side of nearly everyone who actively cares about the balance of Gensokyo. Though there are more reasons than that as to why she wouldn't like him. On a side note, Yukari must be interested in this random human who wandered all the way from the furthest corner of Gensokyo to the human village in a day and seemingly recruited a Youkai as a helper.