Public land encroachment - is there anything I can do?

[Writer]

We moved into our home in Boulder County in the middle of last year. The back of the house abuts public park space, and the fenceline of every home on my block borders the park space at the same point.

We just received a notice in the mail that 5' of our backyard (fence included) is encroaching on the public park space, and they expect us to "fix the problem" (at our expense, I presume) by the end of April.

I've gotta say, if our fence is encroaching into the public space by 5', then EVERYONE's back yard is encroaching by 5'.

This seems so silly... it's not like we're going to continue moving the fence back even more and take more land :P

What's going on here, and what would be the best steps to handle the issue?