Google has been experiencing great market attention to it's Chromebooks. They had the best selling laptop on Amazon this past Christmas. Why?
1) People are buying them as "safe" easy to maintain platforms for performing "browser" based tasks without the (much) threat of OS attacks.
2) I bought one for my Mom who only uses a browser. She loves it. And there are fewer worries about "infections" of malware and the sort.
3) Google KNOWS many of us buy these cheap laptops (Acer's $199 c720 - very nice for the $$) and others for use with Linux. They encourage it. Some of the instructions on how to put Linux onto it come directly from Google.
Why does Google encourage us Linux-heads to buy their cheap box and put Linux on it? It helps them create credibility in the market by selling more Chromebooks. I'd wager to bet that their Chromebook sales would be at least half (remember 44.7% of stats are made up) if not for us Linux-geeks.
Computer manufacturers don't like Linux in general. They cannot simply download and install Ubuntu and slap a "for sale" sticker on it and go. Nope. There is software engineering from the BIOS (EFI), ACPI, and validation and verification. All very costly aspects. My speculation is that El Goog is tied-in closely to the computer OEM's SW-eng design loop taking up slack with V&V too.
One day, when Google has achieved OS Dark Overload status, they may remove the Linux capability. Until then it's win-win.