OG-21 （没有找到先前的相关解释）

Defense attorneys have occasionally argued that their clients' misconduct stemmed from a reaction to something ingested, but in attributing criminal or delinquent behavior to some food allergy, the perpe­trators are in effect told that they are not responsible for their actions.

请问，but表达的转折意思体现在哪儿呢？前后不都是（为）perpetrators在开脱罪名么？

多谢赐教！