When Eliezer arrives at Rivka's home he removed the muzzles from his camels so they could eat. Rashi explains that the Torah deliberately calls our attention to this detail to emphasize that Avraham’s camels traveled with their muzzles on to prevent their eating neighbors’ crops.
Another opinion in the Midrash challenges this view. Certainly Avraham’s camels were no less holy than the donkey of Rav Pinchas ben Yair, which refused food which did not have ma’aser properly taken. There was no need to muzzle Avraham’s camels because they could be relied on not to eat stolen food.
In defense of the first view, the meforshim on the Midrash and meforshei Rashi explain that while the camels could be depended on to miraculously not eat stolen food, that does not absolve Avraham and Eliezer from taking proper precautions. Ain somchin al hanes – we do not rely on miracles, even ones that we know will occur. But why then does the second view argue? What is the point of disagreement between the two sides?
The gemara tells us just as Hashem protects R’ Pinchas ben Yair’s donkey from sin, kal v’chomer Hashem protects tzadikim from coming to sin. Tosfos (Gittin 7a) quotes Rabeinu Tam’s view that this special protection applies only to ma’achalos asuros, but not to other sins. Other Rishonim disagree (see the Otzar Meforshei haTalmud). Perhaps the debate between the two views in the Midrash hinges on this issue. Stolen food is not the same as ma’achalos asuros, as the food is intrinsically permitted; it is the prohibition of theft which places it off limits.
If you want to go a step further, perhaps you might argue that the debate arises specifically in this case where there is an act of achila but an issur gezel (or mazik) – is extra protection afforded because a ma’aseh achila is being done, or does it depend on the chalos of the issur, which is gezel? Along these same lines, Tosfos explains that there is no protection afforded where the issur involved is only derabbanan. Perhaps this indicates that an issur derabbanan done b'shogeg is not an issur at all (because lo tasur is violated only by an act of deliberate rebellion), or perhaps one might explain that even though the act done is one of achila, since the issur violated is the more general lo tasur, no protection is afforded.
The question of how to understand the exemption of ones arises in a number of sugyos. Is an issur done b’ones considered a ma’aseh aveira for which there is no punishment, or is it not even considered a ma’aseh aveira? Rav Yosef Engel (Beis haOtzar #25) writes that a nafka mina between these views would be whether Hashem protects a tzadik from an issur b’ones. I wonder if I am missing something, because it seems that there is a major limitation on this nafka mina. According to Tosfos, Hashem’s protection is limited to cases of ma’achalos asuros, yet misa’aek b’chalavim v’arayos is chayav because of the hana’ah received – i.e. even if one accidentally b’ones ate ma’achalos assuros, one would still be chayav. Of course Hashem’s protection would extend to a case where the act results in a chiyuv. I’ll leave it to the comments for someone to work out a chiluk between ones and misasek (Shu”t RAK”E #7?) or come up with a case that is ones but not misasek.