In the (self-proclaimed) Communist states the formal excuse to ban labour unions was AFAIK that they were unnecessary since, you know, the regime was supposedly "by the workers for the workers"...

Anyway, you're taking things a wee bit for granted methinks. The capitalist system is just as capable of short-sighted and self-desctructive excesses as any other, and insofar as the nasty side effects do not actually affect its fundamentals the matter will only change through deliberate action. The steps taken to block the malign tendencies will then have to be duly updated to keep pace with developements like any other legislation and institution.

This is however somewhat beside the point, which is that as such both the Capital and the Authorities tend to happily let things go to Hell for the workers so long as they can still maintain their position regardless. The Capital can always come back in the form of another enterprise replacing one that went bankrupt or whatever, and if the Authorities rely on force instead of actual legitimacy in the first place (as is more or less the case in China for example) or otherwise can afford to largely ignore the woes of the Labour they tend not give a hoot, especially if they're reimbursed for looking the other way.