Page and Plant were both enormously in love with the blues, much more than their British rock peers, and especially more than the beatles, who treated the genre as a cheap gimmick that they could mine for the occasional riff or lyric. The moment that cemented the decision to form Led Zeppelin came when Jimmy Page invited Robert Plant over to his house to check out his record collection, and Robert Plant already knew and could sing all the obscure Delta bluesmen that Page worshipped.
The Zep founders had an encyclopedic knowledge of the blues before the band was ever founded, and I think that to them when they covered “You Shook Me” or “I Can’t Quit You Baby” it was very obvious what they were doing. So much that they didn’t think it necessary to credit Willie Dixon as the original artist. I admit this is not an entirely good reason, but a lot more generous than the “greedy dishonest rock star” accusation that the Zep detracters drag out so often.

Another factor is that the Zep versions are usually entirely reworked and share little similarities with the original. Compare their “When the Levee Breaks” with Memphis Minnie’s. Both great songs, but as different musically as night and day (although they actually gave her credit on that one). The same with “In My Time of Dying”, “Nobody’s Fault But Mine”, “Whole Lotta Love”, “Travelling Riverside Blues”, etc. The great similarity they share with the original blues staples is the lyrics, and that brings us to the next point.
The era of Blues that Zeppelin so frequently borrowed from was a communal genre more akin to folk music than pop music. Artist borrowed from each other all the time without considering it plagiarism or dishonest. Credit wasn’t important and copyright laws unheard of. Zep gets a lot of flak for stealing Robert Johnson’s “Squeeze my lemon” line, but Robert Johnson didn’t come up with it either. Lyrics about Killing Floors, Backdoor Men, Hoochie Coochie Men,Sugar Mamas and all manner of unfaithful women appear over and over again by all sorts of Delta and electric blues artists. Such was the blues.

It’s also important to note that whenever Led Zeppelin was sued for royalties they quickly settled out of court. They never denied their influences. Compare this to the “Ice, Ice Baby” fiasco, or George Harrison’s “My Sweet Lord” reaction.

Also, Led Zeppelin did give credit on occasion. There is the above-mentioned “When the Levee Breaks.” There is also a funny anecdote about “Boogie with Stu,” from Physical Graffiti. They tried to give Ritchie Valens’ mother credit because they borrowed lyrics from one of her son’s songs, but she tried to sue them for the whole song instead. Wikipedia has a brief summary of the event, but it’s missing Robert Plant’s really hilarious version of the story.

In conclusion, the members of Zep were not perfect, but for me they will always be the greatest band of all time, and even that is an understatement. They contributed far more than they took from the genre that inspired them.

The one thing I’ve never been able to reconcile about Zeppelin is how they rarely credited the artists they sampled from. I’ve always thought it was cool to hear echoes of Howlin Wolf or Willie Dixon, or to discover where the riff for ‘Moby Dick’ came from.

They freely used what they liked and made something new and exciting with it. But they had the money to pay for royalties, so I’ve always wondered why not give credit where credit is due?

The main riff from ‘Moby Dick’ is inspired by ‘Watch You Step’, whereas ‘I Feel Fine’ is a lazy, blatant rip-off. Just more empirical evidence to support the argument that Led Zeppelin and the beatles are on opposite ends of the musical spectrum, with the beatles occupying the Unbearably Sucky side, and Zep dominating the Unmitigated Awesome side