It is pretty suprising!. There's no strong evolutionary reason for being able to taste the difference: there's no natural source of such concentrated D20 and it's only toxic if you ingest a lot of the concentrated stuff. It's not something any animal is going to encounter in nature.
And by conventional chemical wisdom D20 and H20 are virtually identical: the electron structure is the same and that basically dictates the chemistry. The only significant difference is the mass of the molecule (about 10% heavier), and experiments with oxygen-18 water (which has the same mass as D20) showed it doesn't taste of anything, so it must be due to very slight changes in structure between the two.