Basic Lossless compression can function like this:
Imagine a string of 1's and 0's e.g. 10010000011000101111001
This string can be trivially compressed in a losses manor using this algo, every time the bit changes to a one or zeor, record the previous run of bits. So we would compress this string to look like: [1,1][0,2][1,1][0,5][1,2][0,3][1,1][0,1][1,4][0,2][1,1]
I understand now this is a terrible example however it is good enough here.
So since most/all compression algos are just using a pre-defined set of choices on how to compress data in either a lossless or lossy manor. The only difference between two compression algos would be their rules for what data to keep and how to arrange it more efficiently into a different data structure.
I think the analogy of board games can be used here. While you are free to get a trade mark on many aspects of your game, you can not patent the actual rules or game play. http://www.copyright.gov/fls/fl108.html
The rules and game play is what makes Risk different from mouse-trap.
Replace rules with compression algo (or any software algo...) and we come to the conclusion of software is not patentable.
So at what point does a collection of fundamental computer science concepts become patentable?
The line is completely arbitrary using the compressions algo example. I honestly have not been able to reason through a real life example that hold up to this scrutiny.
Maybe I am misinterpreting your answer, so if I am, I apologize, just ignore me =D
EDIT
Thanks to person for the discussion free down vote. Why engage when you can suppress.