Why does EVERYONE follow Reimu's danmaku battle rules no matter the circumstances?
I know that most residents of Gensokyo follow the rules to be able to fight with the shrine maiden on equal grounds without risking the Hakurei border, but what about Utsuho when she was trying to invade and let hell's fire loose on the above-ground world? Against Reimu, Utsuho might be following rules to keep the border. Against Marisa... I don't see any reason.
Maybe Yukari was involved in making sure that the danmaku battle rules are observed???
The spell card rules are (probably? Given the circumstances that led to their creation) written on a vampire contract. A vampire contract is unbreakable, thus all are bound to follow it and (probably?) couldn't break it even if they wanted to. The original contract is mentioned both in Perfect Memento and in Cage in Lunatic Runagate (the Yukari chapter, where Yukari briefly mentions youkai can't eat native humans under the contract), although the spell card rules would have been more an addendum/revision/revised version of the original contract..
It is implied that an unknown youkai was the original writer of the spell card rules in Perfect Memento, and Yukari is listed in Perfect Memento's credits as one who worked with vampire contracts. Thus it is highly likely that Yukari was indeed involved. But the power of the contract comes from it being a devil contract, not Yukari (though Yukari, as "Sage of the youkai" and "Representative of Gensokyo's reality", was possibly the one with the authority to sign such a contract). The spell card rules came about because of the vampire incident, which is generally believed to have been orchestrated by Remilia.
The general story thus goes like this:
The vampires (presumably the Scarlet sisters, as Akyu states in another section that "when one refers to the vampires in Gensokyo, they must mean the Scarlet sisters" or something like that) invade Gensokyo.
An unknown youkai (presumably Yukari, since she's confirmed to have worked with vampire contracts) defeats Remilia (note: The wiki has a mistranslation that indicates it was Reimu that defeated Remilia during this time. The actual translation allegedly should state that a powerful youkai did from what other Japanese speakers told me. I'd edit it but I don't speak Japanese and thus wouldn't get it very accurately, myself).
An initial contract is drafted with Remilia to prevent such things from happening again. The contract stops Remilia from attacking humans in return for humans from the outside world as food, but apparently includes terms that are insanely harsh and restrctive on youkai in general in Gensokyo (according to Reimu's section).
The youkai object and bring their qualms to the shrine maiden (presumably Reimu, since it's in her section)
An unknown youkai (again, presumably Yukari) drafts a new contract in the form of the spell card rules (but purposefully words the contract as if it was Reimu that wrote it) and gives it (sneaks it?) to Reimu. (given that Yukari and Reimu never really met in person before PCB, it is unlikely the two collaborated together on this. However, apparently there were some discussions amongst youkai and/or Reimu, as other rules were considered) Reimu goes along with it cause the current situation is very very boring for her.
The rest of the youkai presumably gleefully accept this new contract because it isn't a huge nazi fascist restrictive piece of evil unlike the old contract, despite it still being quite restrictive to youkai.
A new golden age in Gensokyo flourishes as a result.
Just as planned.
