Why do people get so angry at someone havinge a 'spare' room ?

Even the supporters of the spare bedroom subsidy have to answer the questions.  What about people who have no money to move - how does leaving them with less money help free up 'spare' rooms ?  What about the people who want to move but have no smaller accomodation to go to - how does that free up spare rooms ?  What about the people who live  in properties adapted to cope with disabilities - how does that save the tax payer money ?  And why the obsession with bedrooms ? what about the other rooms in the property ? Why should a couple and child people living in a house with two bedrooms, garden, garage, conservatory, kitchen, sitting room, bath room, loft, cellar, utility room and get full housing benefit, and an identical couple  in a 3 bedroom high rise flat, including two box rooms and a single open plan living area  and shower/toilet get their benefit cut ?