Difference between AにB and AはB


The context is this tweet:

> **セットリストに適当** ！って書いてます。笑

I could understand more easily if は was used instead of に. How does に change
the meaning?

Does an inference have to be made here to understand what is appropriate?




I think this person (maybe a musician) just means:

> セットリストに「適当！」って書いてます。笑
>
> I just write 「適当！」("play without a plan!") on a [set
> list](http://en.wikipedia.org/wiki/Set_list). lol

