This is a good stategy to mitigate old hardware not yet receiving a deactivation signal.
Why this has no impact, I have 2 radios that have never been deactivated and I've let them play through several of these previews. When the previews is over those radios just keep on working. The problem with their system is after a ESN is removed from their system if the radio was never deactivated they apparently have no way of doing so. I have even gone a far as calling them. I was told well that ESN isn't in our system so we have no way to do anything with it. Really? I would imagine there are thousands of radios out there that are still activate that they can't turn off. I've had several over the years. I gave an uncle of mine and old radio setup for his home use. He's been using it for 2 years and though I don't agree with not subscribing it was his choice to continue using it. He said if they turn it off he'll subscribe, if they don't they are idiots and don't deserve the subscription.
I was told a long time ago that Sirius only sends that deactivation signal to the radio for about 7 days. Don't know if that is true, but given the number of radios that I've seen and had experience with myself I'd say that is pretty accurate.
I recently purchased a turner for my JVC head unit, it came in the box already activated. I thought well this is a free preview period as I had never seen that. Well guess what it is still working and that has been since March. When my current sub on my Skydock comes up in July I will switch it over.