Hello Bazzmund.
I understand that people keep domesticated rats as pets - our daughter did for a while. However, in the wild they are vermin, carry disease and are not desirable companions in a domestic environment. When you've had a big one run over your feet in the garden you'll probably appreciate this bit of the discussion. (At the time we lived in a house surrounded by farm land and fully expected and accepted that we would see occasional rats. That didn't mean we should live with them). It is illegal to return a black rat or fat dormouse to the wild.
Grey squirrels are a totally destructive force and have arguably been one of the reasons for the decline of the native red squirrel and one of the reasons for the loss of native bird life too. There is a list of other reasons for the undesirability of the species in the UK. That's the reason why, if caught under any circumstances they cannot be returned to the wild and that is a matter of legislation.
I could go in to reams about the American Signal Crayfish. The native white clawed crayfish and bullheads which I used to catch and keep to study are almost extinct as a result of the introduction of this species. The Signal Crayfish is another species which it is illegal to return to the wild and is caught frequently by anglers in the UK.
There are any number of similar examples - we can even blame rabbits on the Romans - but, and in this I agree, this is not the forum for such discussions. This is presented as a balancing argument
I write this as someone who has lived in what could arguably be called the country for the whole of my married life and worked with farmers and countrymen outside my business career. This could easily become a heated discussion and is anyway probably outside the boundaries of WT - not that, subject to us all playing nicely, any matter of interest is beyond our bounds. No matter where this discussion goes now I will comment about this issue no further as I recognise there is a real risk that discussions about such matters can easily become inflamed, a situation I wish to avoid.
Brian