This has nothing to do with anything, it's just interesting.
Assuming (like Reb Elchonon in the beginning of Kesuvos and the Chasam Sofer in the first teshuva in CM) that the logic of the petur of אונס is that your act is not attributed to you, then why are we rewarded for fulfilling Mitzvos if we are motivated by a fear of something worse than פחד מיתה- a fear of the punishment for transgressing Hashem's commandments which just starts with מיתה. If you did the Mitzvah because you are an oneis, then you didn't do the mitzvah at all. If without the oneis you wouldn't have done the Mitzvah, then you didn't do the Mitzvah at all.
יראת העונש is a perfect way of describing the din of אונס. In fact, as you know, the Gemara calls the acceptance of Torah that was extracted by כפה עליהם הר כגיגית is called מודעה רבה לאורייתא and would remain so if not for the הדר קיבלוה בימי אחשורוש. But even after the קימו וקבלו, everyone that does mitzvos because of יראת העונש should have a problem that it's not misyacheis to you because of the lomdus of Oneis.
There are three answers to this question.
1. That gufa is what the Torah is mechayeiv. The tzura of the Mitzvos is that you do them אנוס על פי הדיבור.
2. The lomdus that the ma'aseh is not misyacheis to you is that it is misyacheis to the person that caused the oneis. So fine, here the ma'aseh is misyacheis to the Ribono shel Olam, which is the biggest kiyum of the din Shlichus. The less it's misyacheis to you, the more it's misyacheis to the Ribono shel Olam, and the bigger the din of Shlichus is.
3. The Rambam by kofin. Deep inside, we all want to do the Mitzvos מאהבה.
What's the difference? The difference is for a person who never fasts on Yom Kippur. He is a total sheigitz, and he couldn't care less if it's Yom Kippur or Purim. One year, nebach, he was ne'enas, he was so sick that he couldn't eat or drink anything- punkt on Yom Kippur. Does he get the schar of fasting? If the lomdus of Oneis is that the ma'aseh is not misyacheis to you, then his fasting should not be misyacheis to him here, either. Bishlema a person who normally fasts, it's not mistavra that the Oneis will take away from his kavana to fast, because he would fast anyway, as the Beis HaLevi (and Hafla'ah and Pnei Yehoshua) says, and there's no din oneis where you would have done it anyway. But our sheigitz would not have fasted, and he's only fasting because of the oneis, so it should not be misyacheis to him.
According to answer #3, this man would get schar for fasting.