Spike wasn't one of her subjects. Technically, she could summon him just like she could any dragon using the scepter, but ultimately he was Equestrian, and if she tried to boss him around, his big sister with the big wings and big horn would come with a list of objections and the magic to turn her inside-out. In a lot of ways, they were on equal footing, her a ruler and him an adoptive relative of one. He'd been a kid when they'd met, but he'd never been one of "her" kids, not really.