It means you are expecting help from people who are willing to help you, not that you're commanding other people to help you.
What would be rude and commanding would be "Thank you for your help", in my opinion, because it implies the help must occur.
Also, all of these formulations, because they are commonly used, carry a lot of implicit meaning with them, and those meanings may differ for different writers or readers.
For me, for example, "thank you in advance" includes "thank you for reading", "thank you for trying to help if you can", "I'll be grateful if you give me an answer" and "sorry for the time you spend on it" (the last one in all cases).
If someone reading me finds it's rude, he can always suggest me a form that would suit him better, but I would only be careful with my messages to him, not others.