Not to defend the big corporation, but there are a couple of reasons for these currencies that aren't related to "stealing" from players by leaving you with a couple of dollars worth of premium currency after you make a purchase. They're about reducing transaction costs by reducing the number of interactions with financial services providers players make while purchasing small in-game products.
Credit card transaction fees eat up a lot of the revenue for small purchases like $1.50 BG skins. A lot of small stores will not accept credit cards for transactions below a certain threshold, while bigger stores just jack up prices on smaller items to cover credit card costs. Spending $100 on a bundle of currency once and then buying things with that currency a little at a time winds up being much more profitable for Blizzard just because they don't have to process a credit card transaction for every purchase of a couple of dollars worth of packs or skins or whatever.The second reason is to duck the commissions on various mobile app stores, which can reach 30%. If Blizzard can convince mobile players to download HS PC and make their transactions through the PC game instead of mobile versions, it can make a LOT more money selling the same stuff to the same players for the same price.
What is missing from this, and why the anger is justified, is that there is no incentive for the players. If they gave us $10 or $15 in free currency for every $100 we spend on Runestones (without jacking up prices on us), that would go a long way toward calming the community down. As it is, there is no incentive to buy big bundles rather than paying as we go, and there is no incentive to make purchases through HS PC rather than the mobile app, so there is no mechanism for Blizzard to capture many of the best efficiencies of premium currencies.