That pretty much kills Airdrop as an open P2P communication channel (the "undesired" usage) while still enabling one off intentional file transfers (the primary Airdrop use case).
So a 10 minute limit easily kills 99% of "bad" usage without impacting common usage, leading to less resistance/complaints from average users. Killing Airdrop full stop would have greater push back/outrage, with only marginal improvement against the undesired usage.
Q: How does 10 min limit kill the P2P communciation usage?
A: Imagine your cell phone could only receive calls from non-contacts for 10 min. After which you had to remember to push a button to re-allow calls for another 10m, rinse/repeat. With that UX, the chance you ever successfully call or be called by a non-contact is near zero. People wouldn't opt-in frequently enough to have a reliable network.