Pads reflecting in controlled impedance strip?
How can we assume the characteristic impedance of a pad is 50 ohms?


You can't.

There's a similar issue whenever you connect a component to an impedance controlled trace. Whether it's an antenna, a filter, a simple DC-blocking capacitor, or the actual IC pad the trace connects to.


  When the signal transitions from the controlled impedance copper trace to the pads the tuning components are mounted onto wouldn't it be reflected?


If the pad is wider than the track, it will be a capacitive disocontinuity in the track. If your matching network has a capacitor to (AC) ground at this point, then no big deal, you just account for the pad capacitance when choosing your added discrete capacitor. 

If the pad size isn't too much more than the track, and your matching requirements are not too strict, possibly you can just ignore the small capacitance.

If you have stricter requirements, the capacitance provided by the pad might be ameliorated by removing (some of) the ground plane beneath the pad. 

Or by increasing an inductive element in the matching network.

So, short answer: yes, there's likely a discontinuity due to the pad. How you deal with it depends on your requirements, the kind of circuit you're working with, and what freedoms you have to change the layout.