It's a rhythm section thing.

The bassist and the drummer should be locked in with each other. I think that because of the nature of their roles in the band, having to work closely with each other, they're also very close friends as well. During the heated Achtung Baby times, it was Bono and Edge sorta arguing with Adam and Larry.
Some excerpts from Bass Player magazine...2005.
Bass Player: With all the time-sync’d delay and echo effects Edge uses, do you need to have a lot of him in your mix?
Adam Clayton: Larry is always locked with Edge, and sometimes it’s better for me not to hear exactly what Edge was doing, because it would put me in a different rhythmic space. I lock with Larry, and whatever Edge does fits over the top. Now that I can hear much more of Edge, I have to be careful, because I need to stick with what I am doing.
BP: Onstage, are your ears drawn to the drums first?
AC:Yes. The drums tell me everything. Everything else registers a millisecond later.
BP: What are you listening for in the drums?
AC: I can’t say. Miles Davis once said that he likes driving his yellow Ferrari when he gets it up over 70 mph and it starts to hum. It’s something like that. There’s a point—and we’ve only gotten to it from playing a lot—where the forces of Larry hitting the kit and me hitting the bass mesh, and the electronics of both signals blend. Over time we’ve learned how to reach that threshold.
I’ve never really worked with other drummers. But I have done the odd recording session with other players, and none of them seems to have the right foot Larry has. There’s something about where he places the kick drum. There’s an authority to his kick; everything else sits around it. With other drummers, the rhythmic emphasis changes depending on the balance of the kick against the rest of the kit.