Because Obama's eligibility is irrelevant to the charges Lakin was facing.
IIRC, the judge is not allowing Lakin to tell the court (and the jury) the reason he decided to disobey orders (the unreleased birth certificate).
The question is did Lakin refuse to disobey the lawful orders of three superior officers. Why he did it isn't important.
Begging the Question Logical Fallacy
The question is did Lakin refuse to disobey the lawful orders of three superior officers. Why he did it isn't important.
Why he did it is because he thinks the orders might be illegal. To disallow the "why" argument is to disallow his defense, questioning the legality of the orders.
Also, to disallow the "why" is a result of Begging the Question, assuming the orders are legal.
Care to explain an affirmative defense, Professor?