I think the problem as a whole, and I'm not talking about anyone specifically here, but generally Metallica's fans are usually very closedminded to music outside of the box Metallica's supposed to be in that whenever they try something different, people yell "foul" instead of trying to find merit in it. If a group like Pearl Jam or Wilco did an album with Lou Reed, their fans would be "this could be really cool", but I notice a lot of metalheads have this "not metal === crap" mentality that anything besides metal isn't worth listening to (just look at the way they always find a way to bash grunge on "That Metal Show"), I know some Metallica fans who are still anti-Black Album despite that its been 20 years and the album is now a bonafide classic.
And believe it or not, Lou Reed has fans who are snobs and think Metallica is beneath him as well.