I use Sanyo Eneloop rechargeable batteries. They are your best bet.
Not true.
Common batteries CAN be recharged.
My father bought a cheap charger for flashlight batteries back in the 1960s at Rinks.I still have it.Simple and works.It has a current limiter in the form of a small light bulb and a diode to rectify the AC so the batteries “see” a pulsating DC current .It works on flashlight cells and 9v batteries. Ordinary carbon-zinc flashlight batteries could/can be recharged about a dozen times.The important thing is to not run them completely dead and the battery should not get hot.If it gets hot it also means the liquid inside will boil out and the battery will fail.
Excessive charging current will heat a battery and can then cause it to rupture.
Fast charges kill batteries;a battery that receives its charge over 16 hours and gets barely warm to touch will last many times longer than one cooked in a 15 minute charger.
As for what “they” tell you not to do,the reason is as likely to be their concerns about lawsuits and selling more product.
I have lots of chargers.Pick them up at yard sales for cheap.I had the same GE nicads in a 2-way radio for 10 years.Many people can’t seem to discipline their battery use and keep batteries sorted.They would rather buy new and throw away.Of course our whole CONSUMER society is based on constantly replacing usable goods .