It's like that in the USA, where I used to live. I live in Germany now, and I am a salaried employee. The law here doesn't generally allow any non-remunerated work beyond 10%-15%, and your daily work hour average over a 6 month period cannot exceed 8 hours. Anything above that, and they have to pay my hourly rate times a multiplier depending on how far over or if they are weekend/nighttime hours. I believe they're also required to pay for "on-call" hours, whether you are actually called in or not, at a lesser rate. All of this is statutory, not specific to my contract. Not surprisingly, I'm no longer on the pager duty rotation.
Your salary is rated on a basis of working a certain number of hours per week on average as that is a standard that most people in society run by - that number is otherwise meaningless.
Similar to how oil changes are rated between a driving distance or a change by date, or how your milk and bread has a sell by date. These numbers are guidelines that generally signal to people some amount of confidence in a product/service, but do not reflect the reality of use or worthiness.
Any hours past that are unpaid in that regard and effectively reduce your salary.