The law is violated only if the company tries to maintain or acquire a monopoly position through unreasonable methods. For the courts, a key factor in determining what is unreasonable is whether the practice has a legitimate business justification. The Supreme Court has ruled that violations of the Sherman Act also are violations of Section 5, but Section 5 covers some practices that are beyond the scope of the Sherman Act. It is the FTC's job to enforce Section 5. Determining whether a merger will have that effect requires a thorough economic evaluation or market study.
Some cases are easier than others.
The courts decided many years ago that certain practices, such as price fixing, are so inherently harmful to consumers that a detailed examination isn't necessary to determine whether they are reasonable. Other practices demand closer scrutiny based on principles that the courts and antitrust agencies have developed. A practice is illegal if it restricts competition in some significant way and has no overriding business justification. The antitrust laws are further complicated by the fact that many business practices can have a reasonable business justification even if they limit competition in some way. Consider an agreement among manufacturers to adopt specifications that require fire-resistant materials for certain products.