The "danger" is BSD licenses are highly permissive, you can take BSD licensed software, modify it and sell it and never return the changes to the community or release the source to customers.

This means all the work of the OSS community is 'free' for corporations to take.

Coders are not idiots, they know that possibility and still use the license for their open source code. So, "danger" is a silly word to describe this.

So basically, if you make some improvements to something, you get to control those improvements. What a horrible idea that is.

Note: this is clearly sarcasm, and it pains me that I need to put this here, but some won't get it.

Considering it doesn't sustain the free software you needed to improve in the first place, yes, that is a terrible idea.

Eh you are not losing control with GPL. If you choose not to contribute back (when using a non copy left license), you are choosing to not provide control to anyone else.

This is a pretty simplistic explanation, but the BSD license doesn't require you to provide the source to customers. Essentially the argument is that companies will consume FOSS software without contributing back improvements to the determent of the community.

Isn't that really the norm now with hosted applications, if I patch nginx on my server there is no obligation to create a pull request.

It is the norm because most FOSS is not licensed under the AGPL.

In any case, even AGPL does not require you to create a pull request, you only have to make the source available to your users, i.e. everyone who connects to your patched server.

The danger is that it can return to closed source at any time.

This is true for GPL too if the copyright holders (authors or rights holders if signed over) decide to relicense. It has happened many times before. An example would be Nessus.

This wouldn't retroactively change already released code however.

Only the copyright holder can turn a FOSS project non-FOSS. This is true even for the strictest of copyleft licenses (and in fact has happened before).