Evolutionary theory is no more deserving of a disclaimer that any other subject taught in school. Why aren't folks clamoring for a disclaimer on atomic theory? Or Newtonian Physics?
As for your claim "the only explanation..." that is merely your personal opinion.
Kindly direct me to another scientific explanation.
Think it out. Evolution goes to the core of our belief systems - whether we believe in the theory of happenstance or some form of creationism. Many Evo-Reactionaries push evolution as fact because it makes them feel their beliefs about the origin of life are facts and not beliefs. Because of this a disclaimer is need. The disclaimer merely states the truth. Although some Evo-Reactionaries claim differently, science has not proven that God(s) does not exist or the Christian view of creation (or any other religion) is not true.
All the disclaimer does is state the truth and challenge the students to think for themselves – it seems to me only science-fascists would be against this.
There was a time when people that believed the earth is flat could make the same statement. That should make you very intellectually uncomfortable.
I may be wrong, but are you trying to say that all current scientific explanation should be considered fact? About 500 years ago science felt pretty confident in the current scientific theories. Today we know many were flat out wrong. Odds are 500 years from now the same will be true for our current popular scientific theories. Unlike religion, science is every-changing.