Even besides that, the idea that it was or could have been linked to Harry and Thomas's lives was unusual, such that even though Lord Raith knew that it was an ongoing effect, he didn't know it was linked to either of them until they said so in front of him.
And if I remember correctly, Harry has to ask whether linking a spell to someone's life is possible, so again, it does not seem to be an expected norm.
The way I think it works is, enthrallment isn't an effect stemming continuously from the caster -- it's an effect you place on the person being enthralled. One that, effectively, rewrites or changes something in their mind to make them obey you.
So, to my thinking, it's not, "While under the effects of this ongoing spell, you will obey me," but "I'm altering your brain so that you will obey me."