Well I cannot see what they run on their servers, so it is anyone's guess.
At least with native code I can check what is on my hard disk.
And given my experience, it is hardly the case any company outside the big SV ones or whose main business is unrelated to software, that actually bothers to contribute anything back.
i would assume they are not contributing but if i wanted to find out if they did, i'd ask them.
should be fairly easy to validate if you know what they're contributing to.
while i agree with your point about this being sub-optimal for both software-development and free software as a whole, i think it is a separate issue.
At least with native code I can check what is on my hard disk.
And given my experience, it is hardly the case any company outside the big SV ones or whose main business is unrelated to software, that actually bothers to contribute anything back.