I think Ken hAAmer has hit on the relevant point. You are allowed to stay up to six months on each entry, but if you exceed six months in a given year, there may be tax implications, i.e. you may be subject to IRS taxation for having established a presence in the US for tax purposes.

I know most Canadian snowbirds have to be extremely careful on how many total days they spend across the border over a rolling three year term, in order to avoid additional IRS taxation.