I've build, and supported numerous online purchase portals. Even excluding rancid costs like boxing, advertising, unsold product, and logistical concerns, just the operation of your own systems to support sales can alone eat 30%. The smaller you are, the bigger the chunk.
Visa and the other card processors take a big swipe, as much as 5% off each transaction, and there's substantial investment in getting those systems up and running, especialyl if you also take alternate payment methods and a large variety of cards. The servers (and the internet pipe) have substantial costs as well, unless you business is VERY tiny. Handling post-sale purchase support, returns, and more is a burden as well for a lot of firms, though most smaller companies do handle that by simply not having a phone to call and saying "no" to emailed refund requests. Add in just a smidge of advertising, and throw some money at Google, and you could need to move thousandas of units just to recoup that cost.
Apple is providing the hosting, bandwidth, purchase overhead, even sales tax processing for you.
Granted, if you already invested this money, and have the infrastructure in place, 30% may easily exceed you current operating costs, but this hosing system means a lot of start up developers will be moving into this space, and as you read a lot of companies are simply discontinuing boxed products, and even cutting prices in half after giving apple 30% still looks to be netting them overall more money.
Not all companies will benefit from this, but MANY (even most) will. Others will have to adjust their business models, and likely spend more money to get noticed standing outside the store. Software prices are also expected to adjust accordingly, coming down, as boxed costs go away there's more room for margin, and thus heavier competition. If this adjusts enough (and apple shoudl give it incentive to), it could mean Mac software costs less than the same title on PC, and yet still makes more money for the devs, and unless Microsoft releases a similar marketplace, Windows could take a huge hit (if macs cost $200 more than an equivalent machine, but software is a $400 savings, suddenly the value proposition is easier to accept!)