Why does it seem those who preach tolerance tend to be the most intolerant?
Tolerance is "an allowable amount of variation of a specified quantity."
If you use the correct definition, it is simple to understand that certain things are intolerable---i.e., totally unacceptable as a position or theory, and can be loudly denounced by a "tolerant person."
For example, "Shoot 'em all and let God sort it out" is is not a position to be tolerated when dealing with racism.
But to answer your question, tolerant people who seem to be the most intolerant frequently have the intelligence, education, and training to be able to easily identify the ideas and suggestions that haven't, don't, and won't work---or we are just plain stupid and overly macho.
And my experience is that we ae not hard to tell apart.
Edit: there is a concept in production management called "Lot Tolerance Percent Defective." It allows you decide if you want to produce a perfect product (expensive) or one that can be examined and easily fixed before it goes into the box for sale (lower cost) and the buyer still gets exactly what he ordered.
There is nothing intrinsically wrong with "intolerance."