If the only point of the juxtaposition of Har Sinai with the laws of shemita in Parshas Behar is to teach that the details of all mitzvos were given at Sinai, the lesson could have just as easily conveyed by mentioning Sinai in the context of any number of other mitzvos aside from shemita and yovel(Ramban). There must be some more fundamental connection between the two ideas. The gemara (Shabbos 88) tells us that Hashem lifted Har Sinai and held it over the heads of Klal Yisrael and threatened to drop it on them if they did not accept the Torah. From here, says the gemara, "moda'ah rabbah l'oraysa," Bnei Yisrael had an excuse for not keeping Torah and mitzvos, as their initial kabbalah was done under duress. Only later, at the time of Purim, was there a willing re-acceptance. But how can this be? We just read in Parshas Bechukosai all the promises of reward for doing mitzvos and the punishment of galus for disobeying. If mattan Torah was done under duress and therefore the kabbalas haTorah was not binding, how could there be punishment for disobeying? Perhaps this was the question of "Al mah avdah ha'aretz?" that all the Nevi'im and Chachamim could not answer (Bava Metziya 85). They could not fathom why Klal Yisrael was being sent into galus despite the claim of duress that would excuse them from punishment. The Ramban answers (see Parashas Derachim derush 22) that there is a difference between all other punishments and the punishment of galus. Eretz Yisrael is a special gift. You can use duress as an excuse to get out of punishment, but you can't expect to get an extra special gift while making excuses. Our stay in Eretz Yisrael is predicated on our acceptance of Torah, no matter what. I think this is what the opening to Parshas Behar is hinting at. The excuse of "moda'ah rabbah" may work for other mitzvos, but when it comes to shemitah and yovel, mitzvos that depend on kedushas ha'aretz and yishuv ha'aretz, even if given "b'har Sinai," with the mountain suspended over our heads and under duress, there are no excuses.