Germany, known for its strict public order laws, expects commuters to maintain decorum in shared spaces like trains and buses. Activities perceived as excessively loud or disruptive, such as group singing, can draw complaints, especially if they disturb other passengers. Cultural expressions are generally accepted, but they are expected to respect the norms of the space, especially in public settings where quietness is the norm.
Critics, as you mentioned, have expressed outrage over the incident, suggesting that such behavior reflects poorly on the individuals involved and could lead to legal repercussions. Some have even called for revoking the visas or residency permits of those involved, urging authorities to ban such individuals permanently from entering the country. While this reaction may seem extreme, it underscores the tension between cultural expression and the expectations of public behavior in a foreign country.