They take ~30% cut.
Just to have a working patcher, I think that 30% deserves to be in Valve's pockets. Not to mention the early access to new versions.
Instead of downloading 35.99 MiB between 1.0.0 and 1.0.5 using the (dead) rsync patcher, I have had to download 3.44 GiB on a slow, unreliable connection!!! I dread to think what people with download allowances think of this treatment.
1.0.0: 0 MiB Changed, 0 MiB Compressed
1.0.1: 30.78 MiB Changed, 10.56 MiB Compressed
1.0.2: 3.25 MiB Changed, 1 MiB Compressed
1.0.3: 19.86 MiB Changed, 5.46 MiB Compressed
1.0.4: 4.14 MiB Changed, 1.3 MiB Compressed
1.0.5: 50.01 MiB Changed, 17.67 MiB Compressed
Total: 108.04 MiB Changed, 35.99 MiB Compressed
Downloading 5 Zips: 3.44 GiB Compressed