That's not some statement unique to unions. Every organization suffers from the principal-agent problem and every union has a tension between what the union wants, what it's officers want, and what the members want. Also, there's nothing wrong with greed: it's one of the prime human motivators and is encourage in capitalist/individualist societies.
Unions are frequently reasonably well aligned and sometimes not.
There are many solutions like government mandated elections of the officers, rotation of the officers, multiple unions for a given sector or company or factory, etc... Each solution of course comes with a cost including weaker officers coming in, dilution of the power of the union, etc...
This pattern also exists for governments, transnational organizations, corporations, non-profits, etc... and it doesn't make them bad or good or greedy or saintly - it just is the nature of any group.
Unions are frequently reasonably well aligned and sometimes not.
There are many solutions like government mandated elections of the officers, rotation of the officers, multiple unions for a given sector or company or factory, etc... Each solution of course comes with a cost including weaker officers coming in, dilution of the power of the union, etc...
This pattern also exists for governments, transnational organizations, corporations, non-profits, etc... and it doesn't make them bad or good or greedy or saintly - it just is the nature of any group.