We already had a similar question before.

If a number string can be converted to any numeric type, ISNUMERIC returns 1.

Among the known "issues" with this are:

money: 1,,,,,,1 is a valid money value (obviously it does not make sense though)

float: 1e1 is a valid float value

float: 1d1 is a valid float value

IsNumeric is consistent in the way that it verifies that it can convert to any SQL Server numeric data type (which includes monetary datatypes & float).

So blame it on the conversion functions instead:)