That depends on what sort of rehab people are going to and why.
If people go to rehab to recover from some accident or surgery, they probably get internet somewhere in the clinic. Also if they are going to rehab for curing some substance addiction, there is no reason not to get internet.
But there are rehabs that treat people with mental disorders. Ant some of those may be denied internet access for medical reasons, if the doctor says that internet is bad for them.
For example if someone is treated for eating disorders and has many online friends with eating disorders and some forum or chat membership where kids do nothing but discuss their weight, then the doctor may rule "No internet for the patient".
Also if someone has depressions and has tried to do harm to themselves and that patient has online friends who encouraged him to do so or discussed the top ten painless ways to end your own existence, the doctor will probably not permit the person to go online during rehab.
I guess they have like a room provided with Internet connection , so they can use it when they want via that room only , maybe something like small library with few computers , I dunno and maybe it depends on the rehab it self , each one is different , some the connection is available , some are not
Ah. Thanks. :)