The meaning of socialism is the collective ownership and management over the means of production. Or, as Marx puts it "a community of free individuals, carrying on their work with the means of production in common, in which the labour power of all the different individuals is consciously applied as the combined labour power of the community". In other words, there cannot be a socialist market economy, because that implies that different companies own the means of production. Instead of that, socialism is a system where all the producers collectively own the means of production, and all the producers collectively decide how production is directed.
The reason why this is necessary is outlined by Marx in a variety of his works. To put it in a very VERY simplified fashion, what Marx argues is not that market exchange is distorted by capitalists who own the means of production, but that the logic of market exchange itself leads to capitalists owning the means of production. You cannot have a market without exploitation, the two are both sides of the same coin. You can where this can lead by looking at big cooperatives like Mondragon.
In 1975, ETA(p-m) criticized the Social Council, writing that members of this body were apathetic, and it accused the General Assembly of a similar lack of interest. The document charged that, in meetings of the General Assembly, when the membership reviewed annual business plans and made other key decisions, rank-and-file cooperators had no consciousness of what they were voting for and simply rubber-stamped management's proposals. Moreover, ETA(p-m) argued that managers had real property in the co-ops while workers had only juridical property without corresponding powers. It also
charged that work rhythms were faster in the co-ops than in other firms, that cooperators worked an enormous number of extra hours, and that their dehumanized work environment made them apathetic.
In sum, the leftist factions of ETA observed that cooperative workers were isolated from the rest of the local working class and from the labor movement. They also charged that, as the regional economy became increasingly
competitive, the cooperatives became more like private firms; thus, cooperators were subject to the same kind of exploitation as non-co-op workers. This critique became more forceful as the 1973 economic crisis impacted Spain, Euskadi, and the local cooperative economy. It is probably not coincidental that
the Vigor strike occurred over the issue of salaries, since, after accounting for
the high rate of inflation after 1973, workers in Spain had not received a real increase in their wages since 1972 (Albarracin 1987, 50-51). Another criticism was that democratic organs in the cooperatives, especially the Social Council, were ineffective and unable to represent workers. Finally, cooperative management was accused of being paternalistic and of using the ideology of cooperativism to exploit workers.