As a representative of Hostelz.com, which aggregates information on hostels worldwide, I ran some data queries on our data to check that for you.
This is a list of all countries with Hostelling International hostels, followed by the percent of Hostelling International hostels which are listed in either Hostelworld or Hostelbookers:
Andorra (0%), Argentina (90%), Armenia (100%), Australia (81%), Austria (68%), Belgium (85%), Bolivia (73%), Bosnia & Herzegovina (75%), Brazil (66%), Bulgaria (67%), Cambodia (100%), Canada (82%), Chile (80%), China (74%), Colombia (88%), Costa Rica (50%), Croatia (100%), Cuba (0%), Cyprus (100%), Czech Republic (86%), Denmark (8%), Egypt (0%), England & Wales (50%), Estonia (100%), Fiji (0%), Finland (47%), France (3%), Georgia (100%), Germany (23%), Greece (50%), Hong Kong (100%), Hungary (69%), Iceland (41%), India (50%), Ireland (Republic of) (100%), Israel (71%), Italy (72%), Japan (52%), Lebanon (0%), Libya (0%), Liechtenstein (100%), Lithuania (100%), Luxembourg (100%), Macedonia (0%), Malaysia (55%), Malta (50%), Mexico (91%), Montenegro (100%), Morocco (25%), Namibia (0%), Nepal (0%), Netherlands (100%), New Zealand (86%), Northern Ireland (100%), Norway (32%), Pakistan (0%), Paraguay (100%), Peru (50%), Philippines (25%), Poland (69%), Portugal (79%), Russia (82%), Saudi Arabia (0%), Scotland (81%), Serbia (77%), Singapore (0%), Slovenia (78%), South Africa (85%), South Korea (63%), Spain (47%), Sweden (23%), Switzerland (81%), Taiwan (40%), Thailand (72%), Tunisia (50%), Turkey (50%), Ukraine (100%), United Arab Emirates (0%), Uruguay (75%), USA (81%), Vietnam (100%), Zambia (100%)
Worldwide, 62% of Hostelling International Hostels are listed on either Hostelworld or Hostelbookers.
A couple notes about the data, our Hostelworld and Hostelbookers information is up to date since that gets updated through our partnership with those sites once a month. Our data from Hostelling International is not as up to date and is updated more sporadically.
So it looks like about 25% of Morocco's Hostelling International hostels are listed on either Hostelworld or Hostelbookers. The other countries range all the way from 0% to 100%. As Ankur Banerjee mentioned in his answer, Hostelling International is made up of separate organizations for each country. Some HI organizations put all of their hostels in the booking websites, and some don't work with them at all. Most leave it up to each individual hostel to decide if they want to list themselves.
Another interesting statistic that I just computer for the first time (I don't know why I didn't think to calculate it before), but looking at the stats for all hostels (not just HI ones), worldwide 64% of hostels are listed on Hostelworld, and 39% of hostels are listed on Hostelbookers.
So there are many hostels that don't sign-up for booking websites such as Hostelworld and Hostelbookers. Our website, Hostelz.com tries to include all known hostels, including ones that aren't listed on any of the booking websites. If you are specifically interested in just Hostelling International hostels, HiHostels.com is their official website and will have the most current information on those hostels.