Correct, the dear Captain did steal and was not nice to his band. But... he was commercially irrelevant. Zep's worst selling album was 3 million copies. The Captain was not on enough people's radar to ask about his songs' provenances.
Aura surrounding Zep was huge "Beatlesesque". Fans figured they were geniuses pulling each album from the air with no musical antecedents, not that they were better and wider listeners than their 13-18yr old audience.
Actually it could be argued that as Zep appropriated less they became less interesting.
Fleetwood Mac "Oh Well" riff used for Black Dog
Yardbird's "Tinker Tailor Soldier Sailor"= Song Remains The Same
Little Richard's "Oh My Soul" via Richie Valen's "Oh My Head"= Boogie with Stu