As far as I understand it, nobody is immediately removing UK investigators from existing, already awarded projects. Those are going on as planned (I'm part of one myself). I'm not sure it would even be possible to do so under the research programme's rules. What's happening is that people who are just now putting together new groups of investigators in order to send an application into the next funding call are thinking twice about including UK partners in their applications, because it seems like a potential risk to the project.
In general a partner dropping out of a project partway through is very disruptive, so you like to avoid it. You want partners who will still be there for at least 4 years from now: 1 year for the application preparation/review, plus 3 years for the project if the application is successful. Many people are not sure if that will be true of UK partners; can you say, today, that the UK will still be part of the EU funding mechanisms through 2020? I think there's a good chance they will be, even if they leave the EU (they will probably end up joining the research framework a la carte, like Norway, Iceland, and Switzerland have). But I could forgive people for considering it a risk they'd rather avoid.
I am still having trouble seeing, for certain and without ambiguity, that Brext itself implies that UK researchers can't be listed on future projects. is that a straight-line, 100% implication? Everything I've read about Brexit suggests that nobody can predict the specifics about what will happen, when it will happen, or how much will actually happen.
You're correct, basically nobody knows what will happen. That's why many people don't want a UK partner in current applications (or at least prefer a different partner if a suitable one can be found), because they don't want this risk that, in 2 or 3 years, the UK partner might have to withdraw, if the UK leaves the EU research programmes. They might not have to withdraw, too, but nobody knows for sure, so it's a risk. Hopefully by next funding round the UK's intentions/timeline will be clearer, so people will either unambiguously know that the UK will stay in the EU research programme long-term, or that it won't.
Thanks for clarifying that this is just risk estimation based on not knowing what the outcome is. There is also a risk in not including UK institutions- they have some mighty fine researchers there, so not including them might lead to a project not being approved (for lack of continental experts in a specific topic) or being unsuccessful (for lack of execution capabilities).
It's trading a well-understood risk for a one that is not well-understood, hence my comment about it being irrational.
In general a partner dropping out of a project partway through is very disruptive, so you like to avoid it. You want partners who will still be there for at least 4 years from now: 1 year for the application preparation/review, plus 3 years for the project if the application is successful. Many people are not sure if that will be true of UK partners; can you say, today, that the UK will still be part of the EU funding mechanisms through 2020? I think there's a good chance they will be, even if they leave the EU (they will probably end up joining the research framework a la carte, like Norway, Iceland, and Switzerland have). But I could forgive people for considering it a risk they'd rather avoid.