I was working for Tower in Las Vegas at the time. I certainly remember the metal heads there being upset about it.And being a metal fan myself, I watched the Grammys that year and watched the whole thing unfold before my eyes. I worked in three (3) record stores (Record Factory, Wherehouse Records and Tapes and Spirit Records and Video) from 1986 until I graduated from college in 1991, and I sold both Metallica and Jethro Tull albums and can honestly say that there were definitely much better metal/hard rock releases that the academy could and should have chosen in 1988/1989. I can tell you that many, MANY people including Metallica fans that were put off by the Metallica release because of the way it was recorded and sounded (Zero bass and drums that sounded like tin cans!). It wasn't until they released "One" as a video on MTV that the record finally started to get some traction and began to sell at retail. Jethro Tull was coming out of "the woods" or "fish farm" so to speak after a couple of early 80's releases that did NOTHING sales wise and found Mr Anderson in a rather diminished vocal capacity. "Crest of a Nave" and their 20th Anniversary box set certainly put the band back into the public eye for the first time in years so it really didn't surprise me (or Iron Maiden's bassist and self professed Tull fanatic, Steve Harris) that they won the inaugural Grammy for "Best Hard Rock/Metal" performance. A lot of the "outrage" is hindsight at best and revisionist at worst.
The Grammys (and the Oscars, FWIW) often give out what are essentially 'lifetime achievement' awards, and a lot of people --- including Ian Anderson himself, IIRC -- assumed their winning the award was more for their entire body of work than for just that one album.
I also think that with a lot of these awards, they ultimately get voted for by old farts who don't listen to the music but instead vote for the only artist on the list they've ever heard of.