Total guess on my part but the smaller party is “made whole” which is less politically problematic than if a large company put in a bid, regulators block it, and the insufficient fee harming the company is blamed on the regulatory activity instead of the insufficiency of the fee.

Unless you’re asking why a small fee would hurt a small company. In that case my understanding is that acquisition talks really stall the business and defocus the company for a non trivial amount of time, not to mention that you’ve given a rival deep insights into your business that they can then use to go after you.

It gives the regulators more leverage to demand concessions. Then the regulator ends up playing brinkmanship with the acquiring company.

The US now only has three nationwide cellphone networks so there are lots of concessions that can reasonably be demanded. (They should have never allowed the number of carriers to be consolidated down to three, but that's another story).

Because it causes a large amount of money to go to underdog with basically no strings attached. It’s essentially free money to the smaller party which they could use to increase competition.