I think they have to follow the science so cannot say it is ineffective.
However, I am aware of one study from February that showed that AZ is ineffective against the South African variant, which about 10% of cases in France are, I think.
So if the SA variant really takes off in France and the rest of Europe, they may have a problem.
I don't know about Pfizer studies and the SA variant - would be interested to know if anyone knows of any, although I seem to remember it being reported as effective (?) - if so, that may be why the EU seem to be so inconsistent. They want the most effective against the SA variant, which they thought was Pfizer, so they wanted to keep all the Pfizer in the EU and not administer the AZ. Then EMA said AZ was fine, now they don't know what to do.
But politically the low rates in some EU countries mean the EU is under pressure to use all the vaccine they have within their borders, including the AZ
Interestingly, not all EU countries have such low rates, some are quite decent and with second doses being given after 3 weeks for Pfizer as well, which is overall quite a good rate for those countries.