Because that’s strike-breaking. Because you don’t need many strike-breakers to break a strike. I don’t think it should be illegal (barring corruption, of course), but I do think it should be scorned.
I'm relatively certain that I have different views than you on the usefulness and appropriateness of unions in the modern world, but I appreciate your counter-position on the thread here.
And, scorn is certainly merited from strikers toward strike-breakers. Scorn from non-strikers toward strike-breakers is for the individual to decide based on their opinion of the strike and the union striking, obviously.