The Family Educational Rights and Privacy Act (also known as FERPA) is a Federal law that protects the privacy of student education records. In particular, university faculty members must have written permission from a student in order to release any information from that student's education record. This applies also to letters of recommendation. Therefore, if you wish for me (Adam Bowers) to write a letter of recommendation for you, in order to remain FERPA compliant, please fill out the Letter of Recommendation Waiver and submit it to me.
If you are a UCSD Department of Mathematics faculty member, you can download the Generic Letter of Recommendation FERPA Waiver (that has a blank line instead of my name).