Don't Stifle Active Collaboration
The 15-minute rule only applies to the formal ceremony of the daily stand-up. It is not intended to limit communication within the team; in fact, the underlying goal of the stand-up is to identify areas of planning or collaboration that need more attention!
If the team is adhering to the time-box of the formal meeting, and self-organizing to coordinate with one another afterwards, then the process is working exactly as it should. Furthermore, I would interpret the team's urge to collaborate as a sign that your team is committed, engaged, and largely self-directing.
Don't Interfere with Organically-Evolved Processes
More generically, a Scrum Master should be extremely careful not to dictate practices outside the formal ceremonies. The Scrum Master is there to coach the team on how to leverage the formal ceremonies such as the daily stand-up, but has no authority (and no reasonable incentive) to structure intra-team communications outside of those ceremonies.
As an example, a related question asked about the downsides of "chit-chat" on a project. In addition to the answers already posted there, I would add that it is not the Scrum Master's job to manage team efficiency. That doesn't mean you can't address inefficiencies as a Sprint Retrospective or bring it up as a blocker in a stand-up, but attempting to over-structure individual interactions is a common command-and-control smell.
This is an extremely slippery slope. In my professional experience, any authoritarian interference with organic communications within the team damages esprit de corps, and can even lead to a sort of learned helplessness that is much more damaging to the team's productivity than the "wastefulness" the interference was meant to control.