A recent article argues that a lot of them are:
we hypothesize that SPS notification activity is correlated with the amount of protectionism afforded traded agricultural products and with the level of demand for protectionism by agricultural producers. To test our claims, we assemble a dataset recording the annual number of SPS notifications by each member of the SPS agreement between 1996 and 2005. The evidence supports the hypotheses proposed in this paper. Specifically, as the relative rate of assistance to agriculture increases, the number of SPS notifications by the country decreases. In addition, increases in the productivity of agriculture are associated with decreases in the number of SPS notifications.
The empirical evidence shows that WTO members may be substituting SPS rules for traditional forms (i.e., tariffs, subsidies, etc.) of agricultural protectionism. Even controlling for both consumer awareness and a government’s institutional capacity, SPS regulations are more likely to be altered when agricultural protectionism is lower and when the demand for protectionism by agricultural groups increases. The pattern observed in our data is that regulatory barriers on agricultural trade may also be protectionist policies substituted for more transparent trade restrictions
I wonder if another approach to this issue would be to ask scientists to go through the SPS notifications, to determine whether the measures are based on sound science.