While it is an annoying ticket, it seems that at least LAWA PD has been doing this for a long time in the long-term LAX lots. I remember seeing quite a few front-plate tickets when I parked there 5+ years ago.
They do this in Chicago, also. They always have parking cops trolling the lots (long term and short term) handing out tickets or booting cars.
OT, but I received a ticket once for no front plate (had moved from Atlanta, so no front plate bracket) and another one (separate occasion) for expired registration (I was out of town for a week and the sticker had arrived when I was on the road). Paid the first, had the second thrown out.
Back on topic, I'd just call them before you return it to let them know you received a ticket for no front plate. It wouldn't hurt to make a copy of the citation, either. When you return the car, be sure to speak to a manager to get verification that you will not be charged. Get it in writing, if you can. Also, check your credit card statement for charges down the road to ensure they don't bounce it back to you.