There is no evidence that Assad ordered Polio vaccines to be deliberately withheld from rebel areas. This Assad as Htlter propaganda gets a little over the top sometimes. In the middle of a war against a ferocious enemy known to murder government employees (aka "Assad loyalists"), one would expect there would be some gaps in coverage merely from the vaccinators being afraid to enter certain areas, or the difficulty of identifying and reaching all homes in need of vaccines.
There is some evidence to suggest that it is the type of vaccine being used that may have led to the outbreak:
Unfortunately, what UNICEF reports and what gets republished in almost all mainstream media reports regarding the polio vaccine, fail to mention that this is the live oral polio vaccine, which actually sheds the virus, and that the vaccine is known to cause outbreaks of polio and paralysis. The fact that the oral polio vaccine contains “pathogenic vaccine-derived polioviruses” is very well documented in the scientific literature, but seldom, if ever, reported in the popular media. Children carrying around the live virus shed it, and it ends up in the sewage systems.
Are UNICEF Live Polio Vaccines Causing Polio Among Syrians? 1.7 Billion Polio Vaccines Purchased by UNICEF