Actually, there was legal action. The legendary Willie Dixon sued em and won a settlement out of court. The key when covering old blues men is to at least credit them, like the Stones did, when you're ripping them off.
Here's a list of their various degrees of borrowing.
From Wikipedia:
Allegations of plagiarism
When
Led Zeppelin's debut album was released, it received generally positive reviews, however John Mendelsohn of
Rolling Stone magazine, criticized the band for stealing music, notably "
Black Mountain Side" from
Bert Jansch's "Black Water Side" and the riff from "
Your Time Is Gonna Come" from
Traffic's "
Dear Mr. Fantasy". He also accused the band of mimicking black artists, and showing off. This marked the beginning of a long rift between the band and the magazine, with
Led Zeppelin rejecting later requests for interviews and cover stories as their level of success escalated.
[31]
One song from the album, "
Dazed and Confused", was a song originally written by
Jake Holmes on his album
"The Above Ground Sound" of Jake Holmes. Jimmy Page decided to rework the song, but for reasons unknown claimed sole credit for the song upon release, with Holmes having never received any royalty payments for their recording.
[66] Holmes did not file suit over the song, although he did send the band a letter stating "I understand it's a collaborative effort, but I think you should give me some credit at least and some remunity." His letter was never replied to and he did not follow up on it.
[66]
Led Zeppelin II's credits have also been the subject of debate since the album's release. The prelude to "
Bring It on Home" is a cover of
Sonny Boy Williamson's "Bring it on Home" and drew comparisons with Dixon's "Bring It on Back". "
Whole Lotta Love" (
sample (
info)) shared a line with Dixon's "You Need Love/Woman You Need Love". In the 1970s, Arc Music, the publishing arm of
Chess Records, brought a lawsuit against
Led Zeppelin for copyright infringement over "Bring It on Home" and won an out-of-court settlement.
[67] Dixon himself did not benefit until he
sued Arc Music to recover his royalties and
copyrights. Years later, Dixon filed suit against
Led Zeppelin over "Whole Lotta Love" and a large out-of-court settlement was reached. Later pressings of
Led Zeppelin II credit Dixon.
The opening chord progression and verse in "Stairway to Heaven" are nearly identical to the
1968 instrumental "Taurus" by the group
Spirit.
[68][69] Led Zeppelin was the opening act for Spirit's 1968 tour.
[70], three years before "Stairway To Heaven" was written, but nobody in
Led Zeppelin has ever cited influence for the song from "Taurus".