There is more to Maths than 'simple calculation'. My 4-year old GS has just won a Maths prize at school based on his understanding of shapes, graphs, tessellation, symmetry, time, number patterns, ordinal and cardinal numbers, etc. He only started in September and is a July birthday.
Where did he get his understanding of these concepts? By using tablets. Where did he get his love of playing with numbers etc? By using tablets
And yes, he knows his sounds, his letter blends, etc from using a tablet.
I agree that their use needs to be regulated both in time spent and on appropriateness of the apps used. But all the above learning has been completely voluntary on his part and most important of all he has thoroughly enjoyed the learning process.