See Rabbi Mordechai Willig's "Shmittah for the Consumer":
Thus, for the American consumer, the main issue is not the effectiveness of the sale (once one assumes its validity), but rather the question of the status of the produce of non-Jews. With respect to fruit, even if one rejects Rav Yosef Karos; ruling and even the validity of the sale, most authorities permit the fruit to be eaten until "z'man biur" as Shmitah fruit, even if it is worked and guarded as usual (31).
If there was reason for the sale to be done in Israel, then one could rely on it for exported produce as well. (Again, maybe not preferable, but many rabbis will allow it.) Once the sale is done, some allow the produce, some don't. (Keep in mind another reason for the sale was for the economy to survive -- which includes exports as well as local consumption.)
Similarly, there was an OU kashrus alert a few years ago about some American cream cheese with an OU-D that turned out to contain bits of heter-mechira vegetables. The OU's official policy was: "consult your rabbi whether this is a problem", or "avoid this if you don't eat heter mechira"; not "it's not kosher!"