The liberal-minded usually have no problems in spotting abuses of power. And the liberal-minded can usually see straight through the protestations of the abusers.

They can call out the City bankers who abuse the bonus system, regardless of the bankers' charming assurances about the "free market". They can deride the tabloids for their excesses, even when the tabloids loudly invoke "freedom of the press". They can dismiss those justifying misuses of police power, notwithstanding the often alarming claims for the need for "law and order" and "anti-terrorism".

In each of these cases, and in many more, the liberal can simply say: that is an abuse of power, and it matters not how you try to defend it.

However, there seems to be a blind spot for many liberals: unnecessary strikes by public-service unions.

When workers who provide public services go on strike, it is an exercise of power. Of that, there can be no doubt. The question then becomes: what kind of an exercise of power is it?

Any exercise of power can be an abuse of power in certain circumstances. Some may perhaps say that there are no such circumstances: striking public-service workers are beyond criticism. Their unions never abuse their power.

But surely this cannot be a serious proposition. Bosses abuse power; tabloids abuse power; police abuse power. There is no good reason why unions are not capable of abusing their power, too.

So, when is it an abuse of power for public-service workers to go on strike?

There are perhaps two elements.

First, there must be regard to the motivation of the strikers. They may use the language of "health and safety" and "long-term benefits", but it is possible that their motives are primarily selfish and financial. If so, such a motivation necessarily prioritises their personal interests above those whom they serve.

Second, there must be regard to the effects. The adverse impact of strikes by public-service workers is normally most keenly felt not by the strikers – or by their bosses. Nor is it felt by those with resources to circumvent the strike.

In particular, a strike by transport workers is hardly noticed by those with the luxury of being able to work from home or drive in to work. Instead, the effects hurt those who will not be paid if they do not turn up; those whose bosses will insist the day be taken as holiday; and those who may actually lose their jobs.

The direct and immediate consequence of any strike by public-service workers can arguably be worse for certain vulnerable and impoverished members of society than any George Osborne Budget.

But public-service unions seem to get away with it again and again. And they do so often with the silent complicity of the liberal-minded.

An abuse of power is an abuse of power; and selfish motives are selfish motives.

And so, as the London firefighters' union astonishingly threaten a strike [2] on – of all days – Bonfire Night, the liberal must ask the questions: Is this an abuse of power and, if so, why is it being allowed to happen?

David Allen Green blogs on legal and policy matters for the New Statesman. He has recently been appointed a judge for the 2011 Orwell Prize [3] for blogging, for which he was shortlisted this year.