Um, do they not know that the POINT of hiring other artists is to make it sound as near the originals as possible?
When someone licenses music like this, what they're usually saying is, "We want this song included with our product, but we do NOT want to pay as much cash as you're asking." So, the usual alternative would be to not licence the song at all and not use the song. Using a song's background without the lyrics is a way to legally license a song on the cheap, without using the real vocalists.
Getting the vocals to sound similar is fair game. If they weren't allowed to try to get it to sound similar, there would be little reason to license the background at all, unless it were solely for a commercial.
When someone licenses music like this, what they're usually saying is, "We want this song included with our product, but we do NOT want to pay as much cash as you're asking." So, the usual alternative would be to not licence the song at all and not use the song. Using a song's background without the lyrics is a way to legally license a song on the cheap, without using the real vocalists.
Getting the vocals to sound similar is fair game. If they weren't allowed to try to get it to sound similar, there would be little reason to license the background at all, unless it were solely for a commercial.