Not so. You can easily prove that SQL Server does in fact convert blank strings to 0 with the following statement:

select cast('' as int)

ISNUMERIC doesn't work in this case because it doesn't actually try the conversion. It uses a different code path to determine whether a string is convertible to numeric, and that codepath does not follow exactly the same logic as the conversion code path. As Hugo said, this is best characterized as a bug in ISNUMERIC.

For a better example of why you shouldn't depend on ISNUMERIC, try the following two queries:

select cast('.' as float)

select isnumeric('.')

(edited to make final example more clear)