Tevilat keilim is a separate mitzva (neither directly related to kashrut nor to tumah) that is most plausibly compared to the ritual immersion of a person who commits himself to the service of G-d by joining the Jewish people. In a similar vein, a utensil that originally had been owned by a non-Jew, when it now is "consecrated" for use by the "mamlechet kohanim" (Kingdom of Priests) also must first be ritually immersed in a mikvah.
In fact one could argue that for those born into the Jewish faith, the only direct connection they have to the tevilah (ritual immersion) of gerut (conversion) is through tevilat keilim. Keilim (utensils) in general are an extension of the body which is itself a keli (utensil) which can be used for the service of G-d or for mundane/secular matters. When a ger (convert) draws close to G-d by joining His service he is consecrating his body toward that end. Similarly, whenever he acquires a new utensil that was originally from the mundane/secular world, to use in his newly-consecrated life devoted to G-d, he must also first ritually immerse that utensil for that service. (See also http://halachipedia.com/index.php?title=Tevilat_Keilim.)
Since the mitzvah is based on the "consecration" of the utensil for the use of a Jew, it is only binding when the Jew first acquires it for actual use, not if he is merely reselling it. It is only when he plans to use it that it is conceptually an extension of his actual body, and therefore requiring of tevilah.
As far as why there is room for leniency for a guest, see, e.g., http://www.dinonline.org/2014/04/13/eating-from-non-toveled-dishes-2/:
There is a prohibition, in principle, against using dishes that were not toveled (Rema, Yoreh De’ah 120:8). Authorities discuss the nature
of this prohibition (rabbinic or Torah).
This obligation applies specifically to the owner of the dishes.
However, somebody who borrows dishes from a Jew also needs to ensure
they are toveled, and a guest is possible considered as a sho’el.
However, it is possible that somebody eating as a guest is not
considered as “borrowing” the dishes, because it is the owner’s use to
have guests in his home (see Shut Beis Avi no. 116).
Rav Shlomo Zalman adds a point in that if the guest has an obligation
to take the food out of the dish to place it in another (toveled)
dish, he can for the same price place it in his mouth (Tevillas Keilim
p. 86). This will apply specifically to dishes and not to cutlery.
Another consideration is the status of porcelain (china) dishes. These
dishes might be considered kli cheres (earthenware) and therefore
exempt from the obligation of tevillah, though most are stringent to
tovel them because of their similarity (today) to glass. Again, this
consideration will not apply to cutlery.
Therefore, because of the noted considerations there is room for
leniency under extenuating circumstances (though for cutlery one
should try to avoid the use of non-toveled metal cutlery).