Aw. (blush) Thanks, SP.
If the employer were prohibited from ever assigning you to any duty that you were not initially hired for, then how would you ever learn new skills that would allow you to move elsewhere within the organization? Or elsewhere? If you were limited to only the duties you were hired for and nothing else, ever, what's to give you the experience to grow and eventually be promoted? Or move to another company into a higher paying job?
No one ever thinks of that, though. No one ever thinks that if the offer letter is contractual and the employer can't ever ask you do do anything not listed there, then they also can't increase your pay or your benefits; they would also be restricted to what was listed in that offer letter. But for some reason everyone assumes that while they are entitled to increases in pay or benefits, the employer can never change their duties, shift, or job location.
I can't say if writing you up for not performing those duties to standard is over the top or not, since I don't know how those duties relate to what you normally do or what someone in your position might normally be expected to know. But whether it was or not, it was legal.