Why do mobiles phones have non removable batteries at all?
Cost... and you sell more phones since the life is now limited. But in reality there is a significant cost involved in making the case such that you can remove the battery. 

The manufacturer also does the math and considers the life expectancy of the battery vs the life expectancy of the unit itself. With the fast paced improvements in phone generation coupled with the much longer life of modern batteries it came to a point where battery replacement became more or less an obsolete requirement and unnecessary expense.

If you then add to that the cost of supporting distribution of the batteries themselves, which are often custom to the product, it ends up being cheaper to just give the user with the defective battery a new phone.

However, the fact that you can not actually turn the phone off completely, is a worry-some trend.