RMI is a part of Java Core APIs, and it's subject to the same license as JRE/JSDK: you have not to pay any royalty, it's free.

When you downloaded JRE and/or JSDK you've seen a license that is applicable to all the downloaded software, included RMI (inside the JSDK/JRE archives).

Are free all java.* (Core APIs), javax.* (Standard Extensions) packages, they are part of Java, and Java is free.

Strange but true, most of libraries for Java you may need are provided freely: you're not using M$ stuff...