O.K, here's the last word. I work for Betfair (name above is not real!) and was involved in the recent inactive account charge being implemented.
Firstly, we did it because everyone else does. No, that won't placate you but a couple of the posts above have mentioned that online gambling companies aren't banks and that's quite right. There are overheads involved in keeping that collective amount of money safe on your behalf. It certainly doesn't cost nothing to administrate. Additionally, customer funds have to be (by law and for regulatory compliance) ringfenced which means if the company goes bust you still get your money. Unlike, say an Icelandic bank. Anyway, some accounts had literally thousands hidden away from the tax man, ex spouses and so on. Bear in mind, with reference to Betfair, 'inactive' means 13 months or more of doing nothing. Not just not betting, not even logging in, so why should a public company have to shelter those millions indefinitely for no return?
Equally, we had to tell hundreds of thousands of people they had money in their account. Have you considered the flipside of that? Many of those accounts were registered around the grand national or a major football competition and had unclaimed winnings...those people were pretty pleased to get their email and nice, unexpected windfall.
Also, and I can only speak for Betfair here, whilst the fee does filter in the main to the bottom line, it's only when the fee was first introduced that it amounts to a large amount of money (in PLC terms at least)...going forward the sums will be much more in line with what it actually costs to administrate dormant accounts.
But of most importance is that the charge is meant much more to win customers back - Betfair only take the charge if you stay inactive. And you can become active simply by logging in....you don't need to place a bet or spend a penny. In fact you log in, withdraw half the balance and leave the rest for another 13 months when you're account would become inactive again. And even if you do incur the fee, you can claim up to six months of it back as soon as you log back in.
So, whilst I understand a lot of what's been written above my comment, I hope this sheds an alternative light on the situation.