They don't put in the $300 million (give or take) right away. They spend it over time while building the mine. They have to spend a certain amount every year, admittedly small, to keep the option open. They declare their intention to earn-back up to 75% and then have to start spending the money wisely.

They can't guarantee that they can wait up to 3 years in order to buy out CUU and put the obligation to build to rest. Someone else could step in at any time and buy a very attractive option that includes Teck having to fund the mine building while they do nothing. If Teck doesn't buy out the remaining 25% they have to report to whomever does own that percentage and have to arrange financing for the whole operation. Then they have to enter a JV with that company to mine the property.

If they are going for the 75% option, then they would likely arrange the buy-out at the same time of the announcement.