Is it weird that I don't do a courtesy flush at work?
Why don't you give a courtesy flush?
I'll use the bathroom the way I choose to.
You always be considerate of others when using a shared bathroom. Make sure to flush twice if there is a mess in the toilet and always clean up after yourself. If you don't, others in your workspace will likely think you have bad manners.
I don't feel like it was my fault if they don't like to see my shit.
If you don't clean up after yourself, then you can't blame other people for getting upset at your mess. How would you feel if you came into the bathroom and saw someone else's poop in the toilet? Just give an extra flush if the toilet isn't clean yet.