It’s not fine to do them to create, expand, protect, attempt to create, etc. monopolies.
So essentially, you can be anti-competitive only to the point where you’ve been too successful at it, then it’s bad.
Usually due to either a monopsony/cartel/monopoly which controls most of the market doing it successfully.
If the companies in the lower 5% of a market price fix or the like, no one usually cares. Even 20%, usually.
The Sherman antitrust act speaks about ‘restraints of trade’ because it has to actually restrain trade, which requires a significant degree of control - which a successful/actual monopoly, monopsony, or cartel can do.
Technically, even attempting to do it is illegal, but going after every company that tries has a bit of the same feel as locking up every single toddler because they took a swing at someone or threatened them with their cute little stubby kid scissors.
It’s a waste of resources, not in anyone’s interest, stops behavior most people would consider necessary/healthy to some degree, causes much worse problems than it solves, etc.
On the other hand, locking up a successful serial killer is just good public policy.
The difference between the two is more a matter of the success and effectiveness of their tactics, not really intent.