The key here is the Spellcard system which is essentially a magically-enforced contract binding the participants into a "danmaku set to 'stun'" dueling system. Reimu instituted this system because of the way Gensokyo and the Border works; essentially, Reimu functions in the same way as the Pillar of Cephiro, only on an even smaller scale (Gensokyo covering a territory roughly the size of a prefecture). Without the Spellcards, Reimu would pretty much be the ultimate bully, as she could go lethal at will, while her targets would have to pull their punches or risk suffering Sudden Existence Failure. Of course, the fact that certain characters (e.g. Rumia, Flandre, Utsuho) wouldn't understand or wouldn't care to pull their punches is also a significant part of it.

"This is probably a stupid question, but . . ."