I understand if the bitrate is limited to 128kbit for streaming reasons, but if the song is available for download, it should link to a higher quality version of the song (if higher quality is available)

I agree ! I never download some music at 128 berk

I'm pretty sure that t61 site reconverts everything to 128kbps, which would explain it, since it's likely just a link to the server's copy of the song.