Does anyone know which kind of noun 'public' is?
Is it a common noun or a collective noun?
According to Oxford Living Dictionary noun 'public' can require both singular and plural verb forms. If we consider the community as a whole unity, 'public' is used with a singular verb form, for example, the public is very noisy. When 'public' is seen as a group of people, we can use a plural verb form, for example, the public in the bar were watching football. The other examples of collective nouns which can have 'is/are': crew, family, company, etc.
public noun [ U ] [U] Cambridge Dictionary
It is an:
Uncountable or singular noun: a noun that has no plural.
I is not commonly pluralized.
The Oxford English Dictionary quotes a reference from The Times in the year 2000 as follows :
2001 Times 12 Sept. 13/2 Moderate Arab governments whose unfeigned horror at what has happened is dictated as much by anxieties about the impact on their own publics as it is by common humanity.
This agrees with the comment made by @FumbleFingers regarding 'we have different publics'.
And it would indicate that 'public' can be a countable noun.