I've always wondered, what do other countries have to gain by allowing USCBP preclearance in their airport? I kind of get why Canada does it - it allows routes to airports in the US that might not have a USCBP presence, but those flights are short haul, flown by 737s or regional jets and also might make some connections to other parts of the world through Canadian airports on Canadian airlines possible. Ditto for preclearance at the few Caribbean airports that have it, it allows short haul flights to smaller cities (not sure if there actually are any though).

However, I can't imagine a preclearance facility at FRA is going to entice Lufthansa to add a long haul flight to a city in the US so small that it doesn't have it's own USCBP at its airport. Preclearance seems very one sided in favor of the US, I've never understood what Ireland or Abu Dhabi have to gain in this agreement. It's not reciprocal, I doubt the US would ever allow another country to preclear at US arports.

The only reason I can come up with is money, I'm assuming the US pays for the space taken up by a preclearance facility, but I'm sure it's at a discount and probably not as much as they could get from a Starbucks and Cinnabon in the same place.