People with no other options take their first offer. Negotiating is for people with alternatives.

My guess is that they floated the price to a lot of potential acquirers during the poison pill debacle, then they got nothing higher.

New grad with no other offers still negotiate...

Not even negotiating once is just very odd.

New grads with no offers still try, but their only alternative is "I say no and keep looking." Twitter's board's alternative is "I say no and get sued by shareholders." Negotiation only works if you can walk away, and it's not uncommon in business deals to accept the first offer if it is generous enough.