"The highway works as long as nobody drives on it?!"
More than 80% of their speed, more than 90% of the time. So, let's say that 10% of the time it was at or below 80%. Now let's say that a day has 24 hours. So only 2.4 (call it 2) hours of the day was the connection more than 20% below advertised, possibly much more. Now let's say that that occurs when EVERYONE IS USING IT. The system fails under peak load, i'e. when it's being used. So most users will experience failure.
Would you accept internet that is completely nonfunctional from 6-8PM every day? That satisfies your criterion above.
It's not like nobodies using the system when it's down. The system is down because it's being used. This is not an acceptable failure mode.