Porque no los dos? :)

I mean, when you pay fees for routing, you're always paying a specific party for the benefit of connecting two channels: the payment occurs "in" the sending side channel (here, in AB), but that's not the same as "paying for" - you're paying B for their service, not "paying fees for a channel".

So "paying in" is concrete, by "paying for" is almost a philosophical question, albeit a fairly simple one.