Your reaction would have been mine a few years ago. BS is what I would have said too. However...
You easily could have sued. I've seen it done, personally.
In Utah, there was a school district as recent as 2011 that rolled with a no-credit attendance policy. Alpine still does. Many other school districts have already rolled it back. What it essentially is, is if you miss x amount of days/rack up x amount of tardies, you then receive a NC (no credit) for that class until you go to attendance school.
Now, usually the students that really don't care just won't make it up. Most students do care and will make up the work.
This all changed when a certain prominent Utah school was going to give a rich daughter a F for a grade because she had no changed her NC grade through attendance school. Her entitled lawyer father threatened to sue. She had "earned" a C and the school had "no right" to take that "property" away from her based on attendance.
A few other school districts (in other states) have faced this issue, battled in court, and lost. So this Utah school district ended up dropping its entire policy. Guess what attendance is like now?
When speaking with fellow administrators about this (from other states) they know they're one lawsuit away from getting rid of their policy. Many just said, "yeah we know we could be in trouble if anyone threatened to sue. But the benefit is worth the reward. We need to have a strict attendance policy or else truancies and absences go through the roof. We will keep our attendance policy until someone threatens to sue."