Staff actually encourages posting your webcomic at multiple sites.
A lot of creators have their own websites and then post to webcomic portals to increase their traffic. For example, Lumaga has her Four Leaf website, then posts both on Patreon, Tapastic, and LINE. Her Patreon is always the most up to date, followed by her personal website, then Tapastic, and finally LINE last (because she groups multiple pages into larger updates so it lags quite a bit).
The goal is to start off with a wide funnel to capture as many readers as possible and then funnel them towards whatever is the most financially beneficial to your series. In this case Lumaga is taking all her readers from Tapastic and LINE and trying to get them onto Patreon to generate as much revenue as possible.
Personal websites used to be the "go to" for creators but with ad revenue continuing to decline further and further, webcomic portals are starting to become more and more important. With Tapastic adding the Tipping Program some could argue that Tapastic now is more financially beneficial than a personal website. For example with Lumaga and Four Leaf, her personal website is generating only a tiny income from Project Wonderful while her Tapas tipping jar is outdoing PW revenue by a huge margin. LINE generates nothing for her at all directly, but does generate new readers which indirectly can lead to more income. Patreon of course is still doing the best for her but it doesn't generate any leads/readers on its own and needs feeders, so then one could argue that Tapastic is her most important link in the chain of webcomic monetization because it both generates tipping income and new Patreon leads.
Now there are a few situations where you can be embargoed. The first that comes to mind is Hiveworks. If you become one of their creators you can't post that particular series anywhere else. Right now the arrangement makes a lot of financial sense because their ad system (combo of Google AdWords and selling Hiveworks ad contracts) pays remarkably well.
But long term, Hiveworks future is in question because of the ongoing decline in display ad rates across all publishing platforms. Hiveworks actually did the opposite and recently increased by 4x the cost of their ad contracts (2x the price for 1/2 the ads), so for now revenue is up, but its uncertain how long they can keep selling ads at $2 CPM when Project Wonderful has a ton of inventory available for rates much less. Another huge concern for Hiveworks going forward is the lack of a mobile reader. As more and more readers make the shift, it's going to impact their ability to keep up with competitors.
For creators its an easy decision. Stay with Hiveworks and make big ad revenue then jump ship when the tide changes. Since no one is under contract with Hiveworks, and there are no financial penalties for leaving, a lot of creators will probably stay on until Tapas tipping outstrips Hiveworks ad revenue and then transition to Tapastic. There's a ways to go before that happens but as the Tapas usage ranking keeps growing, that time will eventually come.
LINE Webtoons is another that has an embargo. If you become a featured creator you get paid a salary each month and LINE becomes the only place you can post that series. This is still the best deal out there because it can mean $1,000s a month in guaranteed income vs. Tapas tips which there are no assurances and amounts earned can vary wildly from month to month.
MediBang has some featured contests that if you submit, what work you turn in, becomes restricted as well. However posting to their platform outside of a contest has no restrictions, as long as you are not one of their featured creators as well.
"We" refers to Golden Plume Comics as a whole. We currently have one staff writer, two staff artists, and a beta reader collective. However the team is constantly in flux and changes over time as market forces dictate.