“He has no defense.”
Federal law says:
Whoever knowingly and willfully obstructs, resists, or interferes with a Federal law enforcement agent engaged in the performance of the protective functions authorized by this section or by section 1752 of this title shall be fined not more than $1,000 or imprisoned not more than one year, or both.
Lewandowski was fully justified by Federal law to resist Fields’ unlawful act.
You are extremely confused. According to that federal law, Lewandowski could get charged for breaking that law. It was he who interfered with the Federal law enforcement agent performing his duty. He has zero authority to play “insta secret service agent” when he so chooses.