Tranx suggested it originally, but yes I believe he is correct in the assessment that the tube is an un-extruded PET bottle. Just giving proper credit.

I was basing that on un-extruded 2 liter preform I was given in middle school while touring a plastics manufacturing facility.

Also, a good number of PET bottles these days, whether used for soda, bottled water, or any other number of products, use a standard neck and thread size for caps to keep the manufacturing costs down. If you scroll down the Blue Desert Site you'll see an item called Cap2Go, which appears to be the same cap used with Keep2Go and they are holding various water bottles.

It really is marketing genius.Due to the amount of PET bottles used these days, the preforms have to be extremely cheap to buy in quantity. It appears that all Blue Desert did was have caps made with their logo on them. And I'm betting that there's a company out there selling the exact cap they use and offering to add company logos cheaply as well.