OP, you must realise that every child who is late to school has a reason for why it wasn't their fault?
Not my fault my mum forgot to set the alarm, not my fault there was loads of traffic, not my fault my dad couldn't find a parking space. Many times, parents back them up and sometimes you know they're lying.
The school are just trying to send the message, consistently, to every student, that lateness has consequences regardless of reason. Failure to do that would result in a school where dc dribbled in anytime up 9:30 with a plausible excuse.
I teach and do strongly feel that your kids will be better for realising that there are consequences, that they need to take responsibility for themselves, that school rules do not bend for those parents who shout loudest.
It is a bit unfair, if it was your fault, and I guess you owe them an apology but fighting this battle on their behalf will diminish the school a little, teach them little of value, teach them dependence rather than resilience.
Let them do the detention, they'll never forget to set their alarm again.