The usual claim in right-libertarian circles is that it is only possible for monopolies to arise through government action (bribery is sometimes a means to encourage that action, but its not always intentional or that kind of specific corruption, but it is, in most libertarian explanations, always government action.)

And for this purpose, “government action” excludes protection of what the libertarian in question thinks of as proper property rights, which almost dogmatically have no adverse consequences.