New foreign key and merge replication
So it did pretty much what it is described in Make Schema Changes on Publication Databases:


  If a publication is set to allow the propagation of schema changes,
  schema changes are propagated regardless of how the related schema
  option is set for an article in the publication. For example, if you
  select not to replicate foreign key constraints for a table article,
  but then issue an ALTER TABLE command that adds a foreign key to the
  table at the Publisher, the foreign key is added to the table at the
  Subscriber. To prevent this, disable the propagation of schema changes
  before issuing the ALTER TABLE command.


To boot, your exact case is give as an example :) Also, the solution is given there: disable schema changes replication before issuing the ALTER TABLE.

As to why it behaves this way, I would say that a system that would make 'smart' decision what schema changes to replicate and what not would be not only very hard to design, but also very error prone and would cause a lot of surprises and break the principle of least astonishment a lot of times. The gist of it is the relations between objects that are published and objects that are not are often ambiguous or hard to detect (think why is the subject of dependency tracking so vast...). And even when detected, the semantics of a change that affects both a published article and a non-published object are ambiguous to say the least: allow the change only locally? Block the change? I can assure you that whichever answer you venture forward, someone will complain it breaks his case... because the answer is dependent on specific soft business rules, not hard language rules.