They are being acquired for stock, which is why they trade tick for tick with each other. If it was a cash deal, the companies being acquired would be pinned to the purchase price +/- any risk premium.
FYI, I've been trading the USAI- EXPE, ROOM, TREE spreads, here are the stock swap ratios:
EXPE 1.93875
ROOM 2.4
TREE .6199
They are all tight spreads, you've got to be quick.