Did they literaly freeze the cylinder?
This scarily sounds to me that they filled it with CO2
Or you are mistaken and it is a CO2 cylinder and not a 3000psi HPA cylinder.
There is a principle of freezing a cylinder before filling with CO2 for a full fill.
Does your cylinder have a fill nipple as per the picture on page 3 of this:
http://www.oaklandsfestival.host56.com/web_documents/air information.pdf
If yes then you are correct and have a HPA cylinder which should only be filled with 'breathable air', if they have frozen it and filled with CO2 then they have contaminated your cylinder and risked the operating and safety components of your air system. Dont refill with air and inform the site of their error, only competent staff should be running fill stations.