A lot of (young) music fans are not aware that 90% of 20th-century popular music was derived from/copied from/inspired by/adapted from earlier songs and artists.
The idea of a major-label artist copying a previously issued recording's melody or lyric and then facing legal action for it is fairly new. I mean, there were cases of it happening in the 70s (famously, the My Sweet Lord/He's So Fine affair), and then it became a bigger issue in the 80s because of hip-hop sampling and so on.
A legal precedent had to be set for what legally constitutes pliagarism in music. This is extremely hard to determine, and is not even consistent from one musical culture to another. Adding to the difficulty is the historical precedent set by generations of recording artists copying earlier artists and traditional performers and not giving credit. The fact is that a lot of old blues guys, for example, were copied by later recording artists and they never received a cent for it. And until the Me-generation kicked in the 1970s or later, even major-label popular artists would often share and swap songs with each other. (To use George Harrison again, he actually wrote 'My Sweet Lord' and gave it to Billy Preston, who recorded it on one of his first albums. Nobody paid it any attention until a year later when it became an international #1 hit for George, and suddenly there was money to be made.)
Anyway, I just want to underline the fact that absolutely nothing U2 or any other band has ever released is completely unique, and that the precedents -- legally and otherwise -- for what constitutes pliagarism in popular/lyrical music are extremely complex (some would say, convoluted). While most of us (not all, however) in the western music tradition would probably agree that 'My Sweet Lord' is too close melodically to 'He's So Fine' to not warrant some sort of problem, what about other, less-clear examples? How about the Pet Shop Boys' "It's a Sin", which borrows part of the melody of Cat Stevens' "Wild World" -- but does it borrow enough to justify legal action and shared songwriting credits? Half of the people asked would think one way; the other half another way.
But none of this helps the countless artists of the 20s, 30s, and 40s who were ripped off by later recording stars.... they were too far ahead of their time.
Anyway, as per the cited case in this thread -- as others have said -- obviously no small phrase is legal grounds for having pliagarized anything. And obviously U2 didn't invent the phrase. End of.