Why the idea of public sector unions is bad

"Saboteurs stabbed holes in tires of two school buses and deflated tires on 10 others idled by school bus drivers’ strike, authorities and bus company employees said today.

In all, 18 tires were flattened on a dozen buses, said employees of Reliant Transportation’s lot in Greenpoint, Brooklyn."

I guess since they don't get the money they want, they will cost a cash strapped school district to pay even more money.  How does this help their cause in the first place?

Then we have the Union suing it's own members to for "Dues" since Michigan no longer takes the dues out of their members paychecks and hands it over to the Union.

"Big Labor's bad image is well-deserved. Teachers unions mantra is that whatever they are doing, it's "for the kids."
How is suing your members "for the kids?

"The president of Michigan's largest union is instructing officials to prepare to sue its own members, according to a leaked memo issued after the state adopted right-to-work laws in December."