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Command line tool use much more often -x or --foo arguments that a word. Also -S is still faster to type than install, personally I won't switch for 'install'. Nevertheless, you may add some alias to change the behavior if it's annoying you.
I never liked CLI tools that have a "mode" switch that then overloads command arguments. Pacman is especially confusing since -S actually means the sync mode. Which doesn't really fit with the idea of installing a package. Also, why do you use sync mode to install a package, but have a special remove (-R) mode to remove a package? Shouldn't there be a -I to install? To take it further, standard use cases for a packag…
The idea is that you are synchronizing your copy of the local package (which is currently empty/nonexistant) with the remote copy. It's also the same command you use to update that package. So,
pacman -S foo
will upgrade foo if it exists locally, and find and install the most recent version of foo if it does not.
Once you get used to it, the command-line switches in pacman actually address your second concern (the one with apt/yum). In pacman, everything is built on top of a very simple database model - it just so happens that that simple database model is also powerful enough to serve a range of complex functions.