Turkish industrial relations are quite far from my area of competence, but this kind of threat against labour unions, which intend to strike in protest against police violence regarding Takshim square/Gezi Park.
The information here suggests that the strikes may indeed be illegal in the context of that law, but it is hard to say anything about anything regarding legality, considering what the Turkish government does – see this:
“At a mobile clinic on the square, one medic said: “They promised us that they would not attack our field hospital, but they did anyway, firing six rounds of teargas directly into our tent. “This is against all human rights agreements. A serious crime. Not even in war should medical facilities be attacked. But we will remain here and continue our work.””