Basically GLSL specifies that implementations may restrict loops to have "constant" bounds on them. This is to make it simpler to optimize the code to run in parallel (different loop counts for different pixels would be complex). I believe on some implementations the constants even have to be small. Note that the spec just specifies the "minimum" behavior, so some devices might support more complex loop controls than the spec requires.
Here's a nice summary of the constraints: http://www.khronos.org/webgl/public-mailing-list/archives/1012/msg00063.html
Here's the GLSL spec (look at section 4 of Appendix A): http://www.khronos.org/registry/gles/specs/2.0/GLSL_ES_Specification_1.0.17.pdf