The biggest, most criminal, oversight of that era are the scores of great black musicians who invented the music and in many cases had it stolen by record companies and other musicians. While Led Zep, the Stones etc.were doing these mega tours these great artists were toiling away playing hundreds of shows a year, staying in crappy hotels, often segregated, and often working day jobs. Thankfully some white artists did come around and try to make things right by using their fame to highlight their heroes but only a few were able to really get the attention they deserve.
I have to respectfully challenge you here Pete. This is an oft-repeated and generally mis-informed interpretation of what actually went on in that era.
1.) What you say first is true: Blues musicians did have terrible contracts, terrible royalty arrangements, terrible management and did have their music outright stolen from them. What people often fail to recognize though is that this was occurring as early as the late 20s, well before the Rock and roll era. Also, music was typically reworked and appropriated by other black artists in this era. Essentially ALL of the delta blues musicians were stealing from one another.
2.) I'm not sure of what artists you're speaking of but its worth mentioning that, discounting big names like Big Bill Broonzy, Howlin' Wolf, Muddy and others from the 60s Folk Blues revival, who did very well in their era and the rock era that followed, most of the guys were in relative obscurity with a litany of interpersonal problems and alcohol issues way before the 50s/60s era. Lightning Hopkins famously was coaxed from obscurity, reluctantly I may add, by the promise of a bottle of Gin.
3.) I've said this before in conversations on the topic: I am always shocked by how overblown the claims against Zeppelin are. If people can seriously listen to "Lemon Song" and claim that they straight up copped from Wolf and the like, then they aren't listening. Stealing music is one thing. Reinterpreting it is another. Should they have paid proper homage? Yes. This may be their most egregious mistake. Claiming the songs were traditional was a dick move. Should they have to compensate these guys financially for providing them inspiration? Maybe, but they shouldn't be required too. Hell, every Delta, Piedmont, etc musician from the 20s through the 40s lifted music from one another with no change or reinterpretation and never thought twice about it.
Just my $0.02