It's valid to perform research on Earth by taking satellite photos and examining them for land cover use. It's valid to use a laser range finder to map the land elevation of the entire planet. Among the various governments which have done this, Airbus has as well. Presumably to ensure nobody flies a plane into a mountain.

If it's valid to shine a laser pulse from space/the sky onto the entire surface of the Earth (private property included) to determine the elevation of everything, it's valid to send TCP SYNs out to the entire surface of the internet.

That seems ridiculous to me. Is it invalid to conduct research about the colours of houses in my neighbourhood without asking my neighbours first?

That's a poor analogy. The outside colors of the majority of the homes in your neighborhood effectively broadcast that information into the public sphere. A more analogous question is "Is it valid to conduct research about all the homes in the world by around and attempting to peer into all their windows to see if there are any interior bedrooms with pink walls and fluffy animals?"

Alternately, you could ask me "If a computer is broadcasting packets to my computer which is connected to a network port that I'm allowed to connect to, is it ok for me to note that?" In which case I'd say yes.

Scanning through windows to find fluffy animals is certainly getting closer to crossing an ethical boundary but it's not strictly illegal.

I also think that moves the analogy too far in the other direction. While it may not be the case that servers are actively "broadcasting" their port open status to all internet users, that's only because it would not be feasible to do so. It is not because port open status is supposed to be private or secret information, like the inside of my bedroom for example.

In fact TCP/IP is designed with the intention that anyone can check the port open status without authentication. It is part of the intended usage of the protocol, so presumably it is safe to assume that anyone using it is probably using it with that intention.

It's really more like knocking on people's doors and noting the clothing colors of anyone who answers.