This is a very unique category of web-based businesses: ones which keep their promises, but make promises which are unfavourable to their customers/members. The "good for me, bad for you" situation has existed long before the internet was even possible, but web-based operations have several distinct advantages for their owners, with anonymity being the primary one. With a website, it can be nearly impossible to reach the persons behind the website when things turn sour. Fly-by-night businesses are much easier to create and dismantle online. More permanent websites can thrive by slipping their unfavourable terms into a long Terms Of Use policy---which users rarely read in detail.

There is a definite need for warnings about these websites because, although they are legitimate, they can damage an artist's profession to varying degrees (either directly or indirectly). For example, a website which claims rights to artwork being posted on their site can cause copyright issues for an artist who intends to generate income through distribution licenses; a website asking for disproportionate fees for their services can create financial problems for artists who have limited budgets. The wording you want to look for is "worldwide, royalty-free and non-exclusive license(s)"...which means they are allowed to republish and redistribute your content, without limitations and for free. Most of the time, this clause exists so that the website can showcase your work without having to pay to do so. 

When working with online portfolio and sales sites, always make sure that what you're giving them is worth the return.

Resources
American Art Law Organization list at the Artist Help Network
International Art Law Organization list at the Artist Help Network
Art Attorneys list at the Artist Help Network
Canadian Artists Representation Copyright Collective Inc