> **They** were going to hire an assistant for this position, but **they decided** it does not come up often enough to justify the cost.[..] They [..] ask my department's director - **he** was OK with it

Although I personally would appreciate a nice 	diversification of my work, I can perfectly understand why this doesn't feel right.

> I feel that it is bordering on disrespectful

You're treated differently than others. People are deciding about your labour beyond the scope of your contract without your integration. There's not much space for respect in this.

But that's one view on this situation. The other view is your company's. There's one chair which needs to be moved once in a month and there are those young employees who surely have no problem in doing this task.

> Am I over-reacting?

I agree with you that this decision is very unprofessional and it's absolutely valid to feel disrespected. If you don't take consequences, there's no over reaction. Get over it and enjoy the overpayed workout time. 

> Should someone in my position consider manual labor to be an expected part of their job?

You should always consider doing things which are not stated in your contract (e.g. shopping on Amazon). This will make life easier.

I your case I would simply see how things turn out. It might very likely be that your manager's intention has nothing to do with respect. Those extra tasks might happen so rarely that you'll enjoy them plus there will no further situation happen where you feel disrespected. If the future comes differently and you still feel bad you then should seek communication with your director.