Is it fair to give zeros to students who missed early assignments because they added the class late? Last semester, I was teaching a class where
there is a small assignment (~2% of the total course assessment)
that students need to submit every week.
Unfortunately, some of the students joined the class late,
due to add/drop forms that needed to be signed manually,
or for other unknown reasons.

Is it fair to give zeros to students who missed early assignments
because they added the class late?
There were a few students who may have missed 3 or 4 weeks of assignments.

Response to comment

Q: Were the students not able to physically be present?
Were the students not able to predict that they would take your class?

They were physically able to be present.
Some students may not have been able to predict
that they were able to take my class.
For example, one student had to add the class after the add date deadline,
because he was admitted late to the university.

Edit

After thinking things through,
I checked that the last date for adding the course is the Monday of week 2,
and the first weekly assignment is also assigned and due in week 2.
I found in the data that there were several students
who did not attend in week 1,
but all of these students attended and submitted
the weekly assignment in week 2.

Consequently, all of the students in the course
were registered by week 2,
and would have been able to submit all the weekly assignments.
Thus any students who did not submit a weekly assignment
deserve to get zero for that assignment,
unless they have a reason to be excused.
At most institutions, the last day to add a course is set up by the institution, not the professor.  We have an obligation to honor that, which means allowing students to enroll, with full full privileges, up to the last day they're allowed to.

Giving 0's for assignments they've missed is inconsistent with that.  I prefer to simply ignore previous assignments of the sort you're describing - if their score for that part of the class would have been based on 13 weekly assignments, now it's based out of 11.  (Sadly, this can be a little hard to administer depending how you calculate grades.)  Alternatively, they can be asked to make up the work; it is the responsibility of students to keep up with the courses they've enrolled in, either by auditing them while deciding or making up what they missed, so it's reasonable to ask them to do the assignment on a reasonably short time scale (I usually like two weeks, which should be enough time to avoid excessive overlap with any other commitments, but one week would be appropriate if the assignments are short and it's not an exam-laden week).