You may have noticed that a number of mailboxes were added to the capital cities so that, if you happen to find one blocked, it should be relatively easy to walk a few steps or world of Warcraft and find another one that remains unobstructed. Most vendors are located inside buildings where mounts aren't allowed too.
Having the option to use one's flying mounts within the capital cities is ultimately more convenient than removing them to ensure access to services which should be reasonably accessible to begin with.
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