I hate this old argument!
Bottom line like all artists they had influence's
it's just a mere tipping of the hat to other musicians that they like -
and.... it's hard to be "completely" original in a genre of sounds that basically uses I IV V progressions and mostly the major and minor pent
atonic scales (and several modes therein).
and they did do covers as well! a lot of them are stated as so on the album.... they didn't really pull the wool over anybodys eyes.
A LOT in fact i would say at least 40% of what you hear on the radio today is rehashes of old songs, whether they steal just baselines (Sean Kingston - Beatiful Girl, baseline stolen from Ben E. King stand by me) or complete song
melodies and change the lyrics (John Mayer "waiting on the world to change" - melody and chord structure lifted from The Chambers Brothers - People Get Ready)......... i could go on and on about this but i digress...
that's the nature of the beast these days, not many people are truly original artists.