Tax systems are complex. There are deliberate loopholes - a company can register for VAT, for example - and there are unintended loopholes.
Amazon, Google, Starbucks etc engage in very aggressive tax planning. They say it's all legal - they're not trying to evade tax, just avoid it. But because their schemes are so aggressive it might be illegal. We don't really know because the cases would have to go to court. Sometimes tax authorities will investigate, and will persuade a company that their aggressive tax plannin was unlawful, and then get some repayment of the missing tax. This usually means that the company avoids the court case and doesn't have to repay the total tax they avoided.
No-one expects them to pay all their tax burden. People in the UK recognise that tax planning is a useful important activity.
What we don't like is companies taking the piss.
And as to why they should pay tax: they get the benefits from society that flow from taxes - educated workforce, police, road infrastructure, healthcare, etc etc. They should pay their share.
Finally these companies will sometimes mention other taxes. So they'll talk about VAT or PAYE. But VAT is a tax the customer pays. The company collects it, but it's collected from the customer. And PAYE is (apart from the employer's contribution) a tax that the employee pays, again collected by the company. It's a scumbag move to reduce your tax payments to the minimum you can and then talk about the tax other people pay as if it's tax you pay.