Move to unionize charter schools

A NY Times news article poises the following issue:

But the unionization effort raises questions about whether unions will strengthen the charter movement by stabilizing its young, often transient teaching force, or weaken it by preventing administrators from firing ineffective teachers and imposing changes they say help raise achievement, like an extended school year. . . . .

Here is my question: if this really improved the charter schools, why does it have to be forced on the schools?