I don't work for Iberia, but as far as i know it`s specified on their ops manual they cannot give jumpseats to non pilots.
That might be the reason why they denied the jumpseat to to you.
Some of their pilots even take it further and if you are not rated on the aircraft you wanna jumpseat on they wont give you the jumpseat.
I know for a fact that Spanair , Air europa and Air nostrum do give jumpseats to Cabin crew, last time i jumpseated from Madrid to Valencia was about a month ago , it was with Spanair, and there were two air hostesses from Aireuropa jumpseating along in the same flight and had no problems.
In any case it appears that most of the pilotsfrom iberia don't want to bother to give aa jumpseat if you are not working in their company, and pilots from other companies are getting fed up with the situation , because they are repeatedly denied a jumpseat with Iberia, and now they are doing the same with pilots from Iberia asking for a jumppseat in other carriers.
As we say in Spain: "Donde las dan, las toman"
I personally have got jumpseats with several spanish carriers ( so far ,Spanair, AirEuropa and Air nostrum) and had no problem except for Iberia, i tried them several times and was denied it every time, last one, the Captain even made a rude coment about my jumpseating intentions, which i didn't take notice or reply to, but as you can imagine, whenever i am in the situation of deciding if a pilot from iberia is to jumpseat in the aircraft i am flying, the decision will be obviously a big NO.