A narrower grant than full calendar access
Free/busy visibility reveals only whether a given block of time is occupied or available, without exposing what the underlying event actually is, who else is involved, or any further detail. This is a meaningfully narrower grant than full calendar access, and it's specifically what makes cross-organization scheduling practical at all: two people can find a mutually available time without either side needing to expose the substance of everything else on their calendar to the other.
Full calendar access, by contrast, reveals meeting titles, attendee lists, and often the content of what's actually being discussed — information a scheduling counterpart at another organization has no legitimate need to see, and information whose exposure has nothing to do with the actual scheduling task at hand.
Why the granularity has to be designed in deliberately
A system that only offers a binary choice between 'see nothing about this calendar' and 'see everything on this calendar' forces a tradeoff between scheduling friction and unnecessary over-exposure that a genuinely well-designed system doesn't need to force at all. The free/busy level specifically is what most cross-boundary scheduling actually requires, and building that intermediate level in deliberately — rather than defaulting to the simpler, coarser all-or-nothing choice — is what actually resolves the tradeoff instead of just picking one side of it.