The most obvious reason that I can think of is that most of these companies have near-monopolies in many local markets.

The overarching point is that it's a clear good for society. It doesn't have to be fair - we're not required to be nice, or fair, to corporations - they're not people. We're required to maximise the benefit of the citizens in the country. Sure, being 'unfair' to corporations can have negative long term outcomes, which is why you only do it in cases like these: where the outcome is so clearly a positive for the system as a whole.