This kind of nannying can be really expensive to do correctly when trying to build high performance systems -- to the point where it doesn't make sense to punish everyone just because someone who didn't read the manual MIGHT misuse the product. It's not at all unreasonable to mix transactions with different guarantees if they never touch the same data, and tracking that accurately enough without pissing off your customers with performance needs seems like a fool's errand.

Read the CAP theorem, or stick to your single node.