The Constitution does not permit pot smoking. That would be because the Constitution was written to restrain the government. Anything not prohibited in the Constitution is allowed by definition (cf: Civics 101).
She broke the law. You may not like the law, but she broke it and she had been on probation before.
Your main defense of her was this,
And when Grandma goes to jail, I'll bet it will have a great effect on her grandchildren. "Where's your grandma?" "Jail."
As I asked you, should grandmas be exempt from the law?