That may be true, or not as most contracts when I was in full time employment added the requirement to undertake "other duties as requested by the employer".
I shall always remember the day (many, many years ago) when, arriving at the bank's office where I worked a dog had left a rather large, fresh deposit on the doorstep. The manager told me to clean it up as I was the unfortunate who happened to arrive first. I very reluctantly obeyed his request, retching and heaving all the time, but I never once thought to question his order, much as I would like to have refused.
What is reasonable these days though? Should employees refuse to do things they just don't fancy doing?