Hotel - pay cash only query

I was in Rome recently and booked my hotel through one of those 'last minute deals' websites. The booking form stated that I would pay a 10% deposit with my credit card and then the balance in cash.
When I got to the hotel the staff reminded me several times that I had to pay my bill in cash (they seems kind of paranoid about it).
When I paid up on departure - with cash in hand - I noticed that they gave me a handwritten receipt from a receipt book. The couple who checked out in front of me got a computer printout receipt (they payed with credit card).

Now, as far as I'm concerned, cash is king, so I don't mind either way. I'm just curious - is this a regular thing with the 'last minute deal' websites, or was the hotel operating some tax-avoidance operation and keeping guests off the books??

any ideas?

I have 3 different hotels booked in Italy this summer along with a campsite and have been told that I will be expected to pay by cash in all of them though the campsite might accept a credit card! It seems fairly widespread not just limited to last minute sites as I booked in February.