Environmentalist: The United Kingdom recently instituted a law requiring that foods containing genetically altered ingredients be labeled accordingly.
Conclusion
Other countries contemplating such labeling should refrain, because it would necessitate production alternatives that are dangerous and pesticide intensive.
Evidence
The UK required labels on foods with genetically altered ingredients. Producers got rid of these ingredients because they feared consumers would see the labels as warnings.
Many genetically altered crops are pest-resistant, so without demand for these crops, alternative production methods would need to be used.
Evaluate
The environmentalist is concerned that labeling will lead to a drop in demand for genetically altered (a.k.a. pest-resistant) crops, pushing producers to switch to more dangerous, pesticide-heavy alternatives. How's the argument moving from "this happened in the UK" to "so you shouldn't do this elsewhere"?
The argument heavily relies on the assumption that what happened in the UK will also happen in other countries. If producers around the world react the same way as those in the UK, then the environmentalist's conclusion seems plausible.
Moreover, it assumes that consumers interpret these labels as warnings, prompting behavior changes—specifically avoiding the products.
Goal
Look for assumptions that either connect the UK experience with potential reactions in other countries or deal with consumer interpretation of labels as warnings.