I can also see it happening more on larger / regulated / insured exchanges.
Given that exchange insurance only protects against the exchange themselves being hacked, and not against individual accounts being hacked, then the insurance provider should really be charging based on the value of assets the exchange holds rather than the absolute number of accounts. I take the point regarding customer service, but no customer service provider anywhere is charging $10 per month per account, so obviously most of the charge is just pure profiteering by the exchange as you rightly point out.
Thinking about it more, here in the US, banks have been doing charging inactive fees forever.
Makes you wonder if either the exchanges saw that and decided to make some money following their lead OR if there are people from the banking industry controlling some of these exchanges and just want to move them more in line with what they know / how they do business.
If they don't want people who are not actively trading as customers, they only have a couple of options.
They can just close their accounts, which will generate who knows how much grief of people complaining that their account with
BTC0.00000001 that has been inactive for 6 years was closed for no reason.
They can change account levels and costs to make it pointless for small people to stay if they are not trading, but that may stop new people from coming in.
Or they just charge fees till the people they don't want leave on their own.
-Dave