Vizcard,
I don't believe there is any problem immigration wise. If CBP allow admission in B2 status, then they are good on that front.
The plan might well cause your parents tax problems, which they should be aware of.
It is possible to meet the
Substantial Presence Test, yet still be able to to claim a
Closer Connection to a Foreign Country. In that case, they could file a
Form 8840 with the IRS.
However, that option does not exist if the person has spent 183 days or more in the USA during the year.
In that case, the IRS would treat your parents as Resident For Tax Purposes and they would be subject to US tax on their Worldwide Income.
If that is likely to be the case, I suggest they take professional advice on the implications before putting themselves in that position.