[vidarh]

There's nothing illegal about being a monopolist.

However, being a monopolist makes certain things illegal for you to do, and some of those things are things that are easy to suspect you of, but hard to prove you're not doing.

As such, being a monopolist constrains you, as well as invites long, expensive, frustrating probes into your business that creates costs and risk (even if you're not intentionally violating anti-trust law, doesn't mean some business unit isn't inadvertently doing so).

E.g. IBM was under an anti-trust probe from 1969 until 1982 over their mainframe business. The probe ended with the DOJ concluding the case was without merit, but it is widely considered to have had a big effect on IBMs decisionmaking for more than a decade.