Because ensuring your wage and working conditions offer security and comfort takes precedence over temporary inconveniences of other workers. 'Leftists' opposed to strikes on the basis they'd inconvenience workers are essentially opposed to strikes full stop.
Am I not a leftist because I oppose strikes by policemen and firemen as well? Essential government services -- and for millions of people (a significant chunk of them the poor) public transportation is an essential government service -- should not be interrupted because the public employees are on strike. These strikes are hurting people. It's very different from the inconvenience of a factory or even a school being closed by a strike. People depend on these services in their day to day lives, to get to work, to buy groceries, etc.
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