In the industry I work, this sounds like standard practice.
Whether or not an employee 1) passes their probationary period, 2) has it extended or 3) is let go, is weighted heavily on attendance rather than performance.
The process is computerized - it's not personal and there isn't a human being making a judgement on how 'worthy' an employee's absence happens to be or not.
In our system, to pass the probation period employees must meet their targets at a minimum; exceeding probation targets would not offset sickness days. In fact there isn't an option to record 'exceeds', it says "Employee has met all probationary performance targets, Yes/No".
Bonuses are dealt with differently. These are only ever paid to employees who 'significantly exceed' targets for the previous year. Sick days are not taken into account, however an employee is highly unlikely to 'significantly exceed' if they are off work a lot.
When the end of probation reports are being prepared, the reason for sickness absence is not shown (just the number of sick days taken).
I'm not sure it would be of any use anyway, as a doctor's certificate would only be required after seven days. We would not be allowed to independently confirm whether an employee had attended a doctors/hospital appointment/was admitted to hospital.
If there has been over a specific percentage of sick days on the report, the probation period would usually be extended automatically. If it was really bad, the employee would be let go.
The employee who has just passed their probation period and handed in a 2-month doctor's certificate for depression is on dangerous ground. I assume he displayed no symptoms whatsoever up until now? In these exact circumstances, I don't know many employers who would not look to dismiss that employee as soon as possible.
Although our processes are automated and impersonal, in practice my employer has been very good. I've required surgery/reasonable adjustments (equipment and a change in hours) and time off for appointments.
Have a chat with people who have been working there for a while - if they are generally happy then I would be inclined to think you are just going through the standard 'corporate computer process', rather than your employer being a bit rubbish.
I hope you're feeling better now 