Because there could be left over residue from the previous hotel guest who might have cooked meth in it:

Instead of brewing coffee, coffee pots are sometimes used to brew methamphetamine.
[…]
“The coffee makers that you find in every motel room is an ideal heat source. They mix it up in the coffee pot, put it on a heat source and let it sit there and cook,” said Phillips.
[…]
The problem is residue from chemicals such as red phosphorus and iodine.
“Typical sickness and issues that would come with any chemical exposure, simple nausea, vomiting to maybe a hospital visit,” said Phillips.
Phillips says it’s pretty easy to tell if a coffee pot has been used to cook meth.
It will have a dark reddish-orange stain.

(via)