PAL as used in most of Europe uses negative modulation -- and
one reason was precisely that interference is less noticeable,
being black flecks rather than peak white.

"Snow" and "interference" are not the same thing.

I was brought up being taught that interference -- such as spark-plug
pulses -- was more likely to increase the signal level than decrease it.
Therefore, it made sense for the sync pulses to be at peak modulation, so
that interference would not reduce or even erase them.