In general, an industry enters an agreement with a union that represents workers in that field. Part of that agreement outlines the negotiation of grievances. Should those avenues of negotiation breakdown, a union could vote to strike.
When they strike, they do not get paid, although some unions may have saved away money to pay union members during strikes.
Whether it is legal to fire striking employees depends on the circumstances behind the strike. Most strikes occur because contract negotiations have broken down long after the contract has expired. In this situation, a company could freely fire the employees (since they are working without a contract). In a situation in which the union is striking because they feel the company has done something unfair or is not living up to the contract, they may be legally enjoined from firing the workers (depends on the contract).
The problem is that it may not be a viable option to fire that many employees at once, depending on skill level. The cost of a shutdown of business during a strike may be preferrable to hiring, training and integrating new employees. In some fields, and I’d guess that the writer’s strike may fit the bill, it would never be a viable option due to the nature of the work.
Either way, firing striking workers has been seen as public relations nightmare and often backfires in the post-industrial economy.
The National Labor Relations Act broadly permits strikes with exceptions – airline and railway employees are only permitted to strike except within narrow parameters and federal employees waive their right to strike (Reagan fired air traffic controllers in the 80s due to an illegal strike). There is also a clause that bans strikes that would create a national emergency.
This is from a national sense (and only U.S. law). States are free to create additional categorization of workers who can and cannot strike. In many states, teachers, firefighters and police officers are prohibited from striking – although they will often organize union-wide sick days to skirt the law.