On the contrary, a rejection rate that low implies rubber-stamping, prima facie. You would need positive evidence to support your assertion, e.g. that FISA submissions are unusually high-quality. The actual case is, I am sure, that the system was constructed to make allowing the warrant to be easy, rejecting it hard, and the people involved are just responding to incentives. Namely, since it's all secret they are only accountable to each other, so why give each other a hard time?