Part of it is to also draw people in with things. Normally, a lot of zoos I know of add decor for major holidays and hold them as events to draw in more people...or just as a safer place to do things such as the Halloween one where there's an in zoo event that they let the local kids into for a safer option (This year being the exception to it).
This really seems to be more of an american / canadian thing. Except of Zoo Hannover know zoo I know in germany actualy decorates for any holiday. (We only have christmas here anyway, no helloween). I'm with markun on this. As a one year anniversary gift it would be too little for me. We already got some kind of christmas lights last year, so just another type this year would be pretty lame.