Hello , in my opinion, I do not think they put tones in the first place. For Chinese native speakers, it is natural, no need to pay attention. i think what you want to ask should be, why do some words in the songs are pronounced differently in the pronunciation of the speech,am I right?
Chinese natives could understand you largely even if you can't pronounce the tones right. The same for singing in Chinese. Of course some mistakes are inevitable. And we often treat this like a joke and just laugh about it.
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