Circuit breakers cost money and provide no benefit to the park operator, so it makes sense that they would prefer to sell the electricity for a negative price instead as long as that negative price costs them less than the circuit breaker.

Also, solar parks in Europe are subsidized, so beholden to government demands. From the perspective of the government and the public good, it’s better if the electricity is sold for a negative price than if the capacity to produce it for free is wasted, because it can still be used for productive ends. The value for buyers is positive, but because it’s a buyer’s market the electricity is still sold at a loss because the buyers can threaten to go to a different solar park operator.