They need access because they need the data.
Suppose the WHO did recommend a zero travel ban from/to China, what do you think would happen?
Nothing would have changed. It's still in the hands of the politicians in those countries to enact it. And let's be fair, in each country there's at least one politician in the parliament, congress,... that mocked the current, mild, response to the outbreak. Some said they would, or did in fact, defy quarantine rules to stop spreading the disease.
Countries are also running out of reagents to test for this virus so analysis of the situation is already going to be complex. If China is being dishonest, they'll see it in the data. If country X or Y are dishonest in reporting figures about infection, mortality rates and what not, they will also see it.
To summarize and put it bluntly:
1) I doubt China was the only superpower on the planet that used political force on the WHO to change their messaging.
2) The virus is new and currently raw data in any form is more important than issuing a medical recommendation that will be ignored because of country specific politics.