The acronym in the Gemara representing the group of lenient foods regarding the required number of seals is "CHaMPaG". Food items in this group only require one chosam (seal). The "P" in "CHaMPaG" stands for "pas" (bread). Why does bread only require one chosam? The Shach answers that the only concern is that the kosher bread was swapped with pas akum (non-Jewish-made bread). Pas akum is rabbinically prohibited and therefore only one chosam is needed.
Based on this, the Shach says that in a place where the Jewish community has a minhag to be lenient with pas palter (commercially-made non-Jewish bread), kosher bread does not require a seal at all. Even if the original bread had been swapped out, the new bread would be pas palter, which is kosher anyway, so there is nothing to be concerned about.
One could argue that nowadays the assumption of the Shach that non-Jewish bread only presents a rabbinical problem does not necessarily hold true. In the Shach's day, all bread was made from water, flour and a couple of other basic ingredients that were all kosher. But today, other ingredients are also used and therefore it is plausible that the manufacturers could use something that would render the bread non-kosher even from a Torah perspective. So perhaps today, bread should require two seals and not just one?
In practice this doesn't seem to be the case. We see in many kosher supermarket sections that most sliced bread is sold with a mere plastic clamp. Or in the case of special products such as French bread, the bread is stored in a paper bag with no seal at all! Why is this not a problem?
In the case of French bread, there are poskim who rely on the Shach mentioned above to not require any seal because most communities in the US today do indeed rely on pas palter. Regarding the issue of non-kosher ingredients such as cheese or milk being used, this does not apply to French bread, because French bread has a standard recipe that does not allow for them.
Furthermore, each type of bakery bread, including sliced bread, has a unique appearance. So if someone swapped one type of commercial bread with another, it would be quite easy to spot this. In halacha, this use of visual recognition is called "tevias ha'ayin." Even though the Gemara in Bava Metzia says that tevias ayin is only reliable with a talmid chacham, that case is talking about a lost object. But with a food item, even a hedyot (a regular person) could recognize the difference and could rely on this to decide if the kosher bread he is buying is the real thing.
In addition, we are only concerned that the bread would be swapped when the non-Jew is neheneh b'chalipin, meaning that there is something for him to gain from swapping it. For example, if he only has stale non-kosher bread to eat, and we give him fresh kosher bread to watch, he may switch it with the stale bread. Or he may be hungry and not have access to any bread, so he will eat the kosher bread and then swap it later with non-kosher bread. However, in the context of a supermarket where many varieties are freshly-made bread are available, it is not clear how anyone would gain by switching the kosher bread with another bread.
In some kosher supermarkets, the mashgiach of the kosher section will also spend some time going around and ensuring that the kosher bread is not being switched with anything. This, together with this concepts mentioned above, helps us be confident that the kosher bread we are buying is really kosher.
