What is the nature of דינים דרבנן?
In last week's parsha שופטים, we have the din of לא תסור.
There is a well known מחלקת between the Rambam and the Ramban (ספר המצות שורש א) about the nature of דינים דרבנן. The Rambam holds that all דינים דרבנן are based on לא תסור. In other words there is a חיוב דאורייתא to listen to them. The Ramban asks an obvious question, if so why do we say ספק דרבנן לקולא? After all, if you violate a דרבנן you are violating the איסור דאורייתא of לא תסור?
The Meshech Chochma (Devarim 17:11) explains the Rambam as follows. He says that every דין דרבנן is not necessarily a fulfillment of the will of Hashem. The proof is that the Rambam paskens based on the Gemara that a later greater Beis Din can be מבטל a תקנה of an earlier Beis Din. If every תקנה was the will of Hashem how could that be? Therefore, he explains that by דינים דרבנן what is not important is the actual mitzva act, but the fact that you listened to the חכמים and did not rebel against their words. The איסור of לא תסור is an איסור to rebel against the חכמים, to not listen to them. Given that, we understand why ספק דרבנן לקולא because the act of doing the mitzva is not the main point, the point is listening to the חכמים, once it is a ספק, there is no need to do the act because it is not so important (contrast that to a מצוה דאורייתא where the act is clearly an unequivocally the רצון השם).
Based on the above, we can answer another question, do you need to do teshuva for violating an איסור דרבנן. Based on the Mehsech Chochma's understanding of the Rambam the answer would be no. Since the whole idea of דינים דרבנן is to listen the חכמים and not rebel against them as the Meshech Chocham explains, then by definition an עבירה דרבנן בשוגג is not a problem, you did not rebel, you did not know that you were doing an issur and therefore there is no need for teshuva.
The Nesivos סימן רלד סעיף ג also holds that an aveira b'shogeg doesn't need כפרה it would seem for the same reason, that there is no מעשה עבירה.
This idea may also explain those שיטות that hold that a פסיק רישא דלא ניחא ליה is permitted by an איסור דרבנן. Since the whole idea of דינים דרבנן is to listen the חכמים and not rebel against them, a פסיק רישא דלא ניחא ליה is not called rebelling against them, you are not directly doing the action that is prohibited, in fact you don't want it to happen. Therefore, even though it will definitely happen, since you are not doing it directly, it is not an act of rebellion against the חכמים and therefore permitted.