For the minus sign and that kind of cheating, I definitely agree. I think there are at least three distinct reasons to eventually do this though. I'll mention them here for consideration.

- It may be that the student is just using a desktop or online calculator for some trivial part of the exercise. Like in the last step of a calculation, they want cos(2) as a decimal approximation. And they copy the minus sign from there.
- They want to enter the infinity symbol. And they are unaware of "inf" or "infinity". And none of the answer entry help tools are enabled. So they turn to the internet to find an infinity symbol they can copy and paste.
- Combat the perception that "WeBWorK is broken," that it doesn't even "understand minus signs".