Why wouldn’t it be a tax write off? Profits are taxed, not revenues. If revenues were taxed instead of profits, there would be even more powerful consolidation market forces to vertically integrate every single industry to avoid double-taxing revenue.

You can thank the existence of every single independent company to the existence of that tax writeoff you find so unfortunate.

My reading of GP was that it makes it worse because it means there's much less financial reason to avoid making accommodations - which would be somewhat understandable - and that implies they're doing so for other reasons.

Less financial reason, but not much less. 25%-ish at best, or possibly none at all if they're already running at a loss.