Legality of hitchhiking at SAs/PAs in Hokkaido compared to the rest of Japan

I first took up hitchhiking as a primary form of long distance transport back in 2010 in Japan. Since then I have been hitchhiking in Japan on several trips over the years.

For long distances the best places are the SAs and PAs (SA = Service Area = サービスエリア / PA = Parking Area = パーキングエリア). These are located at more or less regular intervals along Japan's expressways, which have high tolls and on which pedestrians are not permitted.

Now in Kyushu and Honshu I've never had any problems hitchhiking to / from / between SAs and PAs along the expressways. It's never considered the same as being a pedestrian on the actual expressway. Many police have seen me hitchhiking at them - I often wave to make sure there's no problem.

On this trip I came to Hokkaido for the first time and naturally I hitchhiked on the expressway as usual. One evening on the way to Sapporo from Hakodate I was at 砂川SA (the northernmost Service Area in Japan) a pair of policemen approached me and told me it was OK to hitchhike but not on the expressway, including the Service Area. I had to go out the back way and hitchhike on the national highway instead (not as fast for covering large distances).

My question is:

• Is it true that hitching at SAs and PAs is OK in the rest of Japan?
• Is it true that hitching at SAs and PAs in Hokkaido is not OK?

Or could it be that these police had an unusual interpretation of the rules and I was just unlucky?

Or could it be that I've always just been lucky hitchhiking elsewhere in Japan and could've been asked to leave the expressway system at any SA/PA in the country?