They should even be allowed to have pens. Why would they need one?
Each ballot has a serial number. After the ballot is separated from the envelope and declared valid, that number must be recorded for a post-election audit.
If you vote in person, the same thing happens. The person who hands you a ballot after establishing that you are a valid voter will record the serial number of the ballot handed to you.
Some idiot probably filled out the ballot in pencil.