In Poland, an English-only worker is a burden to other employees in a bilingual office environment and anyone involved in hiring takes that into account. Basically an English only environment doesn't exist in Poland when Poles are in the majority, they immediately revert to Polish whenever possible (I've seen this in action when the boss didn't realize this was happening when they weren't around).
Then again I don’t know your experiences when you looked for an English only office position job in Krakow or Warsaw.
I would not look for such a position as I am pretty fluent in Polish (and would use my position of being a native speaker of English who is also very fluent in Polish as a selling point - it's not a common skill set). Become fluent in Polish (and understand Polish culture and how to get things done here - which is different from how you get things done in the US or UK) and you can find a position. On the other hand, I'm not interested in office work (been there, done that).
I was just answering your specific question (why advertise in English?). Also, often English is listed as job requirement when the actual job position doesn't require any English (or give the job-holder any real opportunities to use the language if they wanted to, much less require them to do so). Again, the idea is to thin out the applicant pool.