Actually the real reason for this is rather more boring.
Many exchanges now charge 'per person' who looks at the data. Gone are the days of blanket permissions.
If a spread betting company has a client who is just looking at data and not trading then that client is actually costing the company money.
The Fees are not small either, I believe that the LSE has become very greedy indeed charging £4 per month per client (plus an annual fixed charge). So if you log in once a month you will cost a company using LSE data £4 . And the SB companies will be using data from a wide array of exchanges.
If that explained it all (and BTW I'm not strictly saying you're on the wrong path) then surely those who keep their accounts open, and do not even use their data feeds - i.e., they simply keep the account going for say some six months or even a year without doing anything whatsoever with their account - well surely (given your reasoning) such persons should be exempted from the penalty. Why? Because they've done nothing more than leave money idly sitting with them. Such persons are not taking advantage of the data feeds at all, yet are still penalised as if they were!
'Data scroungers/freeloaders', are (just as you say) another case altogether; and especially for a small player (such as ETX), as they just don't have the vast numbers of active clients, your point is perhaps fair enough.
OTOH *if* (as I've suggested) there are too many persons taking advantage of the data feeds lately and not actively trading, then surely more needs to be done to attract such persons to trade actively, as opposed to punishing all outright, and with the same brush.