Do you believe in home bias? That is their Justification for a 45% int weighting.
That's a tough one. First I should point out that I just looked up the Vanguard Total World Stock Index Fund VTWSX and it lists US stocks as 55% of the fund, so if Betterment has 45% international that is market cap weighted, not home bias. I am personally equally split between the US and foreign.
It depends on what economic picture of the universe you believe in. If markets are efficient, then there are two things for home bias: 1) Currency risk beyond a modest amount is uncompensated risk, and 2) in the event of unfriendliness one or more countries may place capital controls on money crossing the border. There is one thing against home bias: something bad might happen to your specific country or its markets, with minimal impact on the rest of the world, or perhaps your country simply fails to return enough to meet your goals while the others collectively do.
So as a simple minded person I say 2:1, if markets are efficient then include international stocks but also have a home bias.
The other economic view of the universe says that bad market events happen often and are worse than would "normally" be expected. Markets are unpredictable rather than efficient. Diversification is the only free lunch, so eat all you can. With this mindset I'd want at least market weight in foreign stocks. There is a lot more of the world "out there" to protect me against the extremely bad downturns which have a good chance of happening. Don't be overweight in a single political and economic system which is heavily concentrated along a tiny section of coastline. Since I overweight international by being 50:50 I guess this might be closer to my view in practice.