I'm no expert (just a lowly
ESL teacher), but I'll have a crack at this one:
The sentence is ambiguous because there is no overt AGENT (because it lacks a "by-phrase"). Lacking the context of discourse, we cannot guess at the AGENT from the sentence alone because the setting seems to narrow to generalize.
Compare with:
A lot of beer is made in Germany.
Even though this sentence lacks a by-phrase overtly denoting the AGENT, the AGENT is understood to be something like
they or
the German people.
This is not the case with your
tables sentence; we simply cannot generalize the AGENT.
What's causing the ambiguity is the use of the "with-phrase". When used with the verb
share, the preposition
with denotes the RECIPIENT, not the AGENT. Therefore, in your passive sentence, we have a THEME (tables) and a RECIPIENT (up to three organizations) but no AGENT. Since we don't know who "does the sharing", we can't make this sentence active.
Compare:
a. passive: The cookies were shared
by Timmy with three kids.
active:
Timmy shared the cookies
with three kids.
(since we have an AGENT:Timmy, we can make this sentence active)
b. passive: The cookies were shared
by three kids.
active:
Three kids shared the cookies.
(this time, the AGENT denoted by the by-phrase is
three kids, so we can make this sentence active).
c. passive: The cookies were shared
with three kids.
active: ???
There is no AGENT (neither overt nor understood) in the passive sentence. Hence, we don't know who does the sharing, and consequently we can't make it active.
The crucial thing to understand is that in (b), the cookies are divided among 3 entities (3 kids) because they play the role of AGENT and there is no other RECIPIENT. On the other hand, in (a), the cookies are divided among 4 entities (3 kids:RECIPIENT + Timmy:AGENT). Since (c) lacks an overt AGENT, we're not only ignorant of
who does the sharing; we also cannot guess at
the number of entities who end up with some cookies:
The cookies were shared
with three kids (
by Timmy? = 4 entities;
by Timmy and his sister? = 5 entities).
In short, I think the source of the confusion lies in the fact that "with" is sometimes erroneously taken to introduce an AGENT in passives (when used in conjunction with verbs like "share"), when its complement actually plays the role of RECIPIENT.
Change it from passive to active 
Linear Mode
