You're correct. There's no issue to sell under a GPL license. The commercial section of WordPress.org are all GPL providers, but many of the largest commercial theme providers are listed there.
ThemeForest actually uses a split license, wherein the templates within the theme are GPL, but the CSS, images, or non-WordPress dependent things in the theme are "protected" under a different license.
GPL has essentially turned into a non-issue for commercial WordPress entities that have gone that route. This doesn't mean they don't get copied and stolen, because it happens all the time - just like non GPL products. But the business models are well beyond selling a template and forgetting about it. Support, updates, reputation, code quality, and other factors are a driving force as to why someone would buy a theme from the actual provider rather than a "legally" ripped version for a fraction of the price.
If ThemeForest uses a split license in the manner you have specified, it falls well within what the Software Freedom Law Center's interpretation of the GPL.
Themes get stolen and copied all over the place. I often have a hard time advising clients to not do that just because it is available. Most of the redistributed versions often contain hidden links and other security issues.
For that and all the points you have mentioned, the price of these themes are nothing compared to the service a good theme developer provides you.
ThemeForest actually uses a split license, wherein the templates within the theme are GPL, but the CSS, images, or non-WordPress dependent things in the theme are "protected" under a different license.
GPL has essentially turned into a non-issue for commercial WordPress entities that have gone that route. This doesn't mean they don't get copied and stolen, because it happens all the time - just like non GPL products. But the business models are well beyond selling a template and forgetting about it. Support, updates, reputation, code quality, and other factors are a driving force as to why someone would buy a theme from the actual provider rather than a "legally" ripped version for a fraction of the price.