When the idea or eclipse plugins are applied in an allprojects Closure then the evaluation of all projects is forced even if configureOnDemand is requested.

include 'a', 'b'

allprojects {
    apply plugin: 'idea'
}
subprojects {
    apply plugin: 'java'
}

When running :a:build with --configure-on-demand the project b is evaluated, too.

Build scans for another example project:
2.14.1: https://scans.gradle.com/s/r2l6bdfgcq5om
3.0: https://scans.gradle.com/s/k6jvlsxn2ssz2

In 3.0, all projects are configured. In 2.14.1, only project1 is configured.

Suspect that it's the idea (and eclipse?) plugin that's causing it. Setting a breakpoint BuildScriptProcessor.execute() shows the Idea plugins doing something with the projects we don't want to be configured.