A plumber isn't spending his hard earned money to plumb and a builder isn't spending his hard earned money to build. They are getting paid as opposed to supporters who are doing the paying, often spending money they can ill afford. Fans who pay their money to travel the length and breadth of the country following the Albion have an absolute right to express an opinion.
Yes, customers do have the right to express an opinion, as long as in this instance they know that doing so is likely to be completely counterproductive and unhelpful if they want the team to win. But why would a customer care, they've paid their money and therefore they're entitled to a good performance and win (apparently). Supporters, on the other hand, will appreciate that there are shades of grey, it's not just 'Yay we're winning' and 'Boo, they're losing'. They appreciate they have the right to boo and moan, but realise doing so won't make the team more likely to win, and instead spurring the team on is more likely to result in a win.
Say you're at work and you've delivered consistently good performances to a customer for a long period, but one day there's a minor mistake which is still fixable. Suddenly, instead of being the jolly, cheery bloke he usually is, he starts sarcastically applauding your co-worker as he leaves the office he's 'useless ay he', and moaning/complaining to the owner because the companies performance hasn't been good enough.
Would that spur you on to deliver a really good outcome with no hard feelings just because he's a paying customer? Or would you bite your lip, think he's a bit of a ****, but just do the job just because that's what you're paid to do and you have no choice?
Would you prefer that he/she understood that there was an error on this occasion, but appreciates that you're going to try your best to fix it? Or would you prefer he bitches, moans and makes fun of your colleagues in order to get the response he wants.
Which approach would make you want to deliver a positive result for him, and do your best for him next time he comes in?