Because they'll be advancing a sustainable future according to their mission statement. The goodwill will come back to them eventually and it is just a good thing to do.
Building brand-only charging networks is not advancing a sustainable future (beyond a certain necessary inflection point) and Tesla should embrace opportunities to move away from that once possible. This is a clear one: adapters people buy with their own money at sites Tesla doesn't pay the electricity for - IMO a no brainer not to go after these. Goodwill. Better future.
IMO they'll get enough first-mover advantage in other ways, than having to start clamping down on adapter users at third-party sites.
Brand only charging networks are sustainable. Brand only charging networks which other brands (which contributed nothing to the installation costs) freely circumvent are not however.
The fact of the matter is that Tesla alone is not able to provide for all infrastructure for all brands. You are ignoring the fact that it costs Tesla money to provide the HPWCs/Clipper creek units and installation, a limited amount of money that ultimately comes from Tesla owners, which the other EV owners contribute nothing to (given Tesla does not get any revenue back, unlike paid locations where the installer might have some ROI).
The AC charging infrastructure can be broken down as follows:
1) Infrastructure provided by other third parties (not car manufacturers): paid for either by being pay per use/subscription, by government, or by location owner. These are intended for all EVs to use.
2) Dealership chargers. The intended use of these vary: only for dealership use, dealership customer use, brand use, or general use.
3) Car manufacturer installed destination charging not at dealerships/car stores (AKA Tesla equivalent Destination charging network).
There are about 13k AC J1772 charging locations in the US that fall under #1 or #2. About 2.8k (round to 3k for illustration purpose) that fall under #3 (I checked the list, it does not include Tesla store locations).
Alternative Fuels Data Center: Electric Vehicle Charging Station Locations
US Tesla Destination Charging | Tesla
In your suggested scenario where such adapters are given free reign (and equivalently in Europe Tesla didn't do software blocking), the way I see the infrastructure develop is that:
13k #1 or #2 locations (+ growth)
3k #3 Tesla (+ growth)
#3 would then be congested, leading to a poor experience for everyone, and the cost will be borne by Tesla. The other EV owners will just use the adapter and there would be no demand for a similar network from their car manufacturer.
In economic terms, I think this issue is called the free-rider problem.
In the other scenario, where there are reserved Tesla-only HPWCs (other than a few goodwill exceptions) and other brand only networks, I see the following:
13k #1 or #2 locations (+ growth)
3k #3 Tesla (+ growth)
Xk #3 non-Tesla (+growth)
In this case, even though a good portion of the overall network is brand only, it's sustainable because there is enough infrastructure to provide for the demand.
Of course the ideal situation is all manufacturers band together and jointly invest in the destination charging network (which is what Tesla suggests for their supercharger network), but that is not the case for Tesla's supercharger/destination charging network at the moment.