If there was any merit to blatantly stealing songs Zeppelin would have been sued and lost residuals as was the case for George Harrison's "My Sweet Lord" which he was found to have plagiarized from the Chiffons "He's so fine"
The "My Sweet Lord"/"He's So Fine" Plagiarism Suit
Also the type of stealing you make reference too well watch this...
YouTube - Pachelbel Rant
It happens, they are 8 notes dude with slight variations things sound similar, also bands cover songs all the time and it's easy to tell what type of music influenced a givens band style, is that stealing?