I once asked a teacher (NY city public school) whether teacher unions were a good idea. He said: "they are bad for you and good for me".
A trade union increases the power of the employees by allowing them to bargain collectively with the employer: when the employer has too much power (e.g., the only job in town), a union can "level the playing field" by controlling a large % of skilled workers and negotiate a better pay or benefits which would otherwise be denied by the employer's access to other labor sources. By controlling the labor force, a union is (or attempts to be) a monopoly itself.
However, in the case of the public employees, like police & teachers, the employer is the public. Thus, from the public's POV, such a union is just another special interest extorting resources from the taxpayers.
The public perception of the teachers and police unions appears to be more benevolent than, say, a commercial interest extracting a tax concession and towards unionization of public employees than other employers.
Why is the public opinion so much more favorable towards the unions of public employees, both compared with the public's attitude towards commercial special interests and compared with the attitude of other employers to trade unions?