I don't think that is the case with the new version of nmap 3.90.
Well, I've been unable to duplicate it...

Maybe its just my version of jetdirect?

nmap -O printerip -p 9100

or with

nmap printerip -p 9100

Seems that the printer will print whatever you throw at port 9100

http://printerip:9100 will print your browser info

------ HP JetDirect Configuration ------
Status: I/O Card Ready
Model Number: J6057A
Hardware Address: 000Exxxxxxxx
Firmware Version: R.25.09
Even your example with 9100 added to the port list doesn't set it off...
Either will --allports

Hmmm