There's some discussion about that going on here:
https://github.com/emezeske/lein-cljsbuild/issues/105 .
The user can, AFAIK, specify the org.clojure/clojurescript version in their project, and that will take precedence over the version that lein-cljsbuild depends on.
I guess my main reason for tying lein-cljsbuild to a specific ClojureScript release is that not all versions of each are mutually compatible. For instance, lein-cljsbuild used to run builds in parallel, via futures, which worked for a while. More recent ClojureScript versions are not thread-safe, though. Another example is that lein-cljsbuild has occasionally set dynamic bindings from the cljs.closure namespace (to configure things in the compiler that are not otherwise configurable). So those versions of the plugin only work with certain ClojureScript versions.
I'm very much open to argument on this topic, though. Pasta knows that I'd like to exit the business of jumping to update the plugin for every compiler release! :)