Transgender

She explicitly said in the letter that she was going to break the law. Here is one of the letters she wrote:

I understand that if you're receiving this letter, you're part of the Florida Bicameral Legislature, which means you're probably one of the people who wrote this law or voted for it. I know that you know in your heart that this law is wrong and unjust. I know that you know in your heart that it's wrong to arrest me and jail me for sixty days for simply using the bathroom. I know that you know in your heart that transgender people are human too, and that you can't arrest us away. I know that you know in your heart that transgender people are no different from you or anybody else. I know that you know in your heart that the same people that go to church with you, eat in the same restaurants, go to the same schools, root for the same sports teams, watch the same movies and pray to the same God as you cannot be all bad. I know that you know that I have dignity. That's why I know that you won't arrest me.

She made an emotional appeal to the Florida legislature and hoped they wouldn't arrest her in an act of civil disobedience. Instead, they sent police to dissuade her from violating the law, and then had her arrested when she broke the law anyway.

If someone sent me a letter saying they were going to use a bathroom I would ignore it, not send cops there.

I guess you're not a Florida legislator, huh? She didn't send the letter to the reasonable, average person - she sent them to the people who voted in the law that bans trans people from using public restrooms. What is relevant here is what the people who did receive the letter would likely do in response?

Your sentence is phrased like they simply had to be there which is just validating, if inadvertently, the idea that going to the fucking washroom should be a crime

void_turtle's phrasing is accurate, the cops were only posted at that particular bathroom at that particular time because she gave advanced warning she was going to be there, they absolutely weren't going to let a trans woman flagrantly violate the law they passed (even if that means enforcing a ridiculous and immoral law - the fact they passed the law is a reason to think they wouldn't mind enforcing that law too).

void_turtle isn't implying this was the right thing for the Florida lawmakers to do, only that it is a reasonable outcome to expect from sending the letter.

she never should encountered any resistance at all, period.

She looks cis passing to me and probably wouldn't have encountered resistance if she hadn't intentionally notified the lawmakers of her intent to violate their law at a particular time and place. That's what got her arrested.

That said, many trans people don't have the passing privilege she has, and the law most impacts those people who anyone would spot as visibly trans, and thus would be at most risk of arrest. Marcy Rheintgen is engaging in civil disobedience she likely wouldn't otherwise be subject to, and it would make a better story of self sacrifice if she wasn't an ignorant reactionary who admitted she didn't actually think she would be arrested and now regrets doing it, lol.