Rush followed Zep by 5 yrs. There's an obvious influence on their first 3 albums (Rush, Fly By Night, Caress of Steel), but it's hardly a rip off. Zep was not above stealing a large portion of their riffs, either.
It's a well-documented fact that Zep influenced early Rush, as well as Skynrd, Cream, amd a few other bands.
As for it being 'allowed'; not sure what you mean. Take A Friend sounds nothing like Whole Lotta Love, imo, even tho the Zep influence is VERY strong, esp with Rutsey playing Drums.