1- The officers won't take your fingerprints because they "feel like it". They follow regulations and one of those is to fingerprint citizens of certain countries.
2- I am a citizen of one of those countries and my fingerprints were taken when flagpoling. The border agents where all professional, polite and friendly as far as a border agent could be.
3- Unless you suspect your fingerprint would be on some "serious" database, the process of flagpoling and if needed fingerprinting is generally EXTREMELY simple and should not be cause of concern for %99.9 of PR applicants.
4- There's a reason there is ADMINISTRATIVE refusal and non-administrative ones. Nobody at CBP woke up one day and thought "How about we make up this random thing and call it admin refusal?". They created the process for a reason and that is to distinguish things such as flagpoling from actual refusal to enter due to issues such as being a convicted criminal.