The Ran seems to understand that although the owner retains ownership of his field during shemittah, he must allow people to enter in order to pick fruits. However, the right of entry that people have on his field is only for the purpose of obtaining the fruit, therefore if they can get access to the fruit without entering the field they do not have a right to enter (As a side point, tosafos seems to indicate that the right of entry is not limited to just picking the fruit, but they can enter and eat the fruit in his field - i don't know if the Ran would argue with that).
However, in a situation where the trees are near the edge of the field so that there is full access without entering the field, the Rosh is not sure whether the issur to enter the field only applies to a mudar hana'ah, or does it apply to all and would constitute trespassing. The second tzad that others may enter, and it is only a mudar hana'ah that cannot enter, is not so clear. If the mitzvah of shemittah allows for complete entry to pick fruits even when they can be obtained from outside the field, the madir should not have the power to assur this on the mudar. And if the mitzvah of shemittah only allows for entry when it is necessary to pick the fruits, why can the owner not block people from entering because of trespassing? Perhaps the Rosh means that although the mitzvah of shemittah does not allow for entry when the fruits can be obtained from outside, we can assume that if it is easier for people to obtain the fruits by entering the owner would not be makpid. Therefore if the owner would put up a sign indicating that he is makpid, it would be assur to enter to take the fruits that can be obtained from the outside even according to the second tzad of the Rosh, just like a mudar hana'ah cannot enter.