But because of patent it doesn't come in some Linux distros by default. In Fedora I had to install the extension for GL myself, in OpenSUSE it doesn't come by default too I think, it used to not come with Ubuntu, now it does (I think). I think that extension is technically illegal under USA law but no patent holder would go after pebbles from suing singular users.
Most of us are not from United States of Patents so for 'us' personally it doesn't matter, especially Laurent is in France (I presume) and look what clusterf**k of patent and DRM breaking software VLC and libdvdcss are - and it's all legal under French law.
Also it'd really really screw what SFML does now, now everything can be loaded in such way : Texture <-> Image <-> File.
GPU only would break that chain. It'd also get into the way of file loading/decoding, SFML uses stbi for loading and writing everything right now, so code for detecting if that's s3tc file and loading it would have to be stuffed near/around stbi calling code that is now in place.
It's really much headache and work for little gain. I'd much rather see the APIs be less airtight so you could use own texture you loaded with GL code in sf::Texture instance and then everyone could do what they want for loading and all.
SFML to me seems hardly about 'options'. I mean, we don't even get anything except RGBA8 for color channel choice.
Either way last word on everything is up to Laurent.
Also, what about the other two algorithms?