That's the thing isn't it, people have spent a lot of money over the years on Sling's products. I'm not sure if they have any idea what they are currently doing with all the goodwill they've built-up.
If they wanted to switch to an ad-based model it was very simple how they should have done it. They should have tied it to the newest M1 Slingbox only, maybe dropped the price, and made it very clear on the packaging/promotion that if you buy this box then you will get ads. Then people could have made the choice. There would still be some grumbling but people wouldn't feel as if it has been inflicted upon them and it would be an active choice whether they want to opt into that by upgrading to the the new box.
To retrospectively add ads to a product that has been sold ad-free from the beginning is at the very least morally suspect, even if not legally.