While I agree that OP's mom is making a big deal over nothing and that the punishment doesn't fit the crime, I'm willing to bet that she's told OP numerous times, finally got fed up, and is doing this to send a message. All in all, if it's her household and she wants things to be a certain way- and OP wants to continue residing there- then putting the seat down is a small price to pay in the grand scheme of things. Similarly, in the future, should she ever visit his place and use the bathroom, she should put the seat back up when finished. For now, OP seems to be learning a lesson the hard way and, through this post, is proving why libraries are still important in this day and age.

So like.. Do most women back into a bathroom and just drop their pants and sit? Because I walk face first into my bathroom.

I'd love to hear the backstory on this!

That sucks. But maybe how many times has they told you to not do it? Only reason that you would be in that much trouble for something so little.

I pressed YDI. Because I have a sneaking suspicion that she may have done this because OP has been ignoring all reasonably worded requests for weeks.

But, you'll never forget to put the toilet seat back down.

I feel like we need to know how long she's been telling you to not leave the lid up before one can judge how harsh the punishment is. If she has been telling you for ages, as parents quite often has to do with their kids, she might just have snapped and done something to force you to listen. Yeah, leaving the lid up might seem like a small thing, but sometimes small things are even better than the big ones at driving you crazy. It's her house too. My mum used to hate when I left dishes in the sink instead of immediately putting them in the dishwasher. I though it was a silly thing to care about, and I always got it done later, so why was she such a pain about it? But then I realized that dammit, she lives there too and needs to feel comfortable, and having dishes in the sink made it look messier so she felt she had to clean them up. I might think it's a minor thing, but she did not, so I started putting them away right away just out of respect. Though as soon as I moved out I stopped, because my house, my rules. Sometimes minor things annoys the hell out of us.

This is completely unreasonable of her.

Thank god tour mom did not take away your library card

That's just cruelty