One thing I've learned about aviation photography is know the law and stand up for your rights! Obviously rules vary from airport to airport but I found this one on the San Antonio airports web site;
Section 3-150 - Prohibition against the Recording of Aircraft Registration Information.
The recording of any Aircraft registration information by Persons or tenants other than those servicing or handling said Aircraft is expressly prohibited without the written permission of the Aircraft owner, operator, or his/her designee.
This can be found here http://www.sanantonio.gov/Aviation/imagelibrary/uploadedfiles/upload-217201130045pm.pdf
I've never had an Officer bring this up but you never know.