A lawyer--progressive, to boot--one explained this kind of thing to me.
Estoppel basically says that if your conduct, behavior, or words allow something, you can't come along later and say that it's disallowed--in the same situation or in others.
It was specifically in regard to a provision in the election code of the graduate student association where I was a grad student. For years a certain leniency in enforcing election regs as stated in the constitution and bylaws had been observed. (Basically nobody who should want the job wanted the job. Deadline for applying and getting your name on the ballot ... Screw the deadline, a couple of hours before the ballot went off to the printers was sufficient, and, hell, they'd hold it if they knew you were running late. Constitution? Bylaws? Sorry, "competent" trumped "on time."
Fast forward to one particular election and a lawyer in grad school for public policy applied late. By 5 minutes. And his opponent, also progressive but quite insanely doctrinaire in his idjithood, insisted that the elections board disqualify him. He didn't follow the rules.
"Estoppel." The lawyer knew that the bylaws were enforced, like, never. Until that year. The lawyers for the association agreed--by not enforcing the "law" the organization had declared the law null. To enforce it now would look like personal animus. The applicant's nomination was accepted.
The HOA where I live posts that the rules are scrupulously and strictly enforced. More on some people and ethnicities than others, but there are random acts of enforcement. Estoppel. If they didn't enforce them (at least from time to time) then they'd be rendered null.
Scoot over to Disney. If Disney didn't enforce its property rights over their characters, they'd lose control over them. You may think that's a good thing. They don't.
The reference to Oregon wasn't incidental. The UO mascot really was based on Donald Duck. Obviously so. And Donald's owner said to stop it--and that produced a firestorm. Disney compromised, protecting their rights and letting the Ducks' mascot continue by setting up a special arrangement.
It's not tasteless, corporate greed. The equivalent for most landowners would be letting, tacitly, somebody set up camp on their land. Then, a year later, finding that they deeded over the land by not requiring some sort of acknowledgement of their rights.
Quack.