Why for some actions wifi is required (ie. cellular is not enough)? Synchronizing iCloud photo library and downloading apps over 100 MB both require wifi, ie. they are not permitted when on cellular.

Is this limit only for "convenience", so there wouldn't be surprises when the phone bill arrives? Is there any official reason? Or technical?

I'm asking this as my current cellular plan (LTE) consistently delivers better speeds than my current ADSL plan (= wifi) and I have plenty of headroom in my transfer quota.
That mechanism is in place to protect customers unaware of the data limits and to conserve the network's bandwidth, this means that everyone gets an equal share of the network. Although what I normally do is USB tether to my Mac then Wi-Fi share the USB connection then connect to that with my iPhone