RPM plugins provide functionality that is not suited to be used everywhere. They may not be built or shipped on some platforms or may not be installed or be disabled on some systems.
This allows plugins to interface with systems that may not acceptable as a dependency for RPM and to provide functionality that may be unwanted under some circumstances.
For now the plugin API is internal only. So there is a limited number of plugins in the RPM sources.
Some plugins can be configured by specific macros or influenced by command line parameters. But most can only be turned on or off. See the plugin's man page for details.
Plugins are controlled by a macro %__transaction_NAME which is set to the location of the plugin file. Undefining the macro or setting it to %{nil} will prevent the plugin from being run.
This can be done on the RPM command line e.g. with --undefine=__transaction_syslog. To disable a plugin permantently drop a file in /etc/rpm/ that contains
__transaction_NAME %{nil}
Another option is to remove the plugin from the system if it is packaged in its own sub package.
For some operations it may be desirable to disable all plugins at once. This can be done by passing --noplugins to rpm at the command line.