it's not plagirism because they admitted to it, and identified that it was written by someone else. it's actually very common for modern (well 1950's and up) to use classical pieces within their songs, two songs i know that use the melody from ode to joy:
The Killers - "Mr. Brightside"
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Bright Eyes - "Road To Joy"
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people don't really call it plagirism when it's an identifiable riff from a classical song, people usually consider more as a little tribute to the original composer.