I can see how directing people to the premium content on entry keeps them from thinking Tapas as a whole is reflected by a single community work, especially when the vast majority of links people will see are from authors looking for new readers. The premium works don't have to advertise to the same degree by themselves, the site shows their work off. On one hand, a community work done in bad taste could really sour someone on the whole platform. On the other hand, the premium works can sometimes have lower writing quality, even if they always look pretty immaculate.
But, the app has a Community tab now, so it seems like a fair trade-off.