Why do Americans think it is fair that because the?

The short answer is we don't. The legislation that forces US expats to file tax returns (although not pay double taxes in most cases) wasn't put to a national vote. We don't like it any better than Boris Johnson does. But it's easy enough to reject US citizenship in the situation you describe by just not registering the foreign birth with the US. Wonder if you'd like to be blamed for every stupid thing your Parliament does. 

It is 100% fair to treat him like EVERY OTHER US citizen.

Boris wanted to renounce his US citizenship he had not lived in the US since he was 5 yrs old, was given US citizenship just because he was born there while his parents worked in the US  so  not something he chose to have and had no intention of living in the US as he had/has a British political career...loyality to the UK was talked about and reality is he would not have become PM without renouncing and making it crystal clear he has zero loyalty the the US... then the IRS billed him for tax on his house sale, purchased from money earned in the UK, lived in in the UK and sold in the UK.... he is taxed by HMRC and the IRS also wanted $50,000 tax paid on it too... which he paid different if he lived in the US even part time and benefitted from the US but he doesn't.

His parents dicision  to have him born in the US, not his and the US birthright citizenship laws...not his, so he made that decision for himself to renounce and ANYONE else with dual US citizenship can pay and renounce their US citizenship in exactly the same way

Different countries have different citizenship laws. It's neither fair or unfair. That's just how it is. It doesn't seem to have ruined Boris Johnson's life.

All US citizens who hold at least one other citizenship are entitled to expatriate.  Whether you think it is it is fair or not, that is the law.  The US is, to all intents and purposes, the only country in the world that taxes its citizens whether they set foot in the country or not.  There has to be some escape from that.

The child of a British mother and an American father born in the UK can decide to give up his US citizenship at the age of 18 without penalty although there are many whose US citizen parents never bother to register the birth so that the US citizenship is just allowed to lapse.  Without a US birthplace nobody is any the wiser.

Citizenship has a price.  If they want to pay that price or not, I don't care.  It is their choice to do so.

Boris Johnson's parents were American?