Am I the only one who doesn't get why Tahm can eat an enemy champ?

I mean think of his lore, he's a Demon River King that strikes bargains then eats everything a few years down the line the poor f*cker who struck said bargain has amassed.

Similarly to Kalista, with her Oathsworn, we can imagine Tahm striking a deal with the ADC if he's played support, and therefore eats said carry in order to profit later on (save carry, don't feed bot, get fed, win game).

So why can he eat an enemy champ that theoretically he hasn't made a deal with?

I'm not complaining, although it's a bit overbearing seeing Tahm every second game on TT, but I've wondered about this since release as it doesn't fit in exactly with his lore.

Unless of course his bargain entailed eating everyone else, in which case I guess it fits o_o