"Ignorance is not an excuse"
It is not anyone's civic duty to know the policies of HS! Op was at an actual facility, not on line. They have to TELL you; that is what the CONTRACT is for.
The cat rescue I used to volunteer for only adopted out sterilized animals. Spay/neuter wasn't even part of the conversation (nor contract) because it was already done. I can easily see the OP leaving with his snake, in good faith, and not thinking this would be an issue.