Why?
Why do Google's accountants have anything to explain? A private company and as such it's in their interest to pay as little tax as is allowable by law. Just as it's in my interest to pay the legal minimum amount of tax. Just as it's in the interests of every single MP on the committee to pay just the legal minimum. I doubt any of them have voluntarily paid more tax than they have to.
Oh dear... a rant is brewing!
And calling it morally repugnant? Please. Since when is it the job of politicians to cast moral judgements? The fact that any politician can believe they are morally superior to anyone or anything is exasperating, by definition that is impossible.
It's morally repugnant to constantly whinge about companies that haven't apparently broken any rules, when they continually fail to change the rules that they themselves are responsible for. It should be MPs in front of a committee.