I can easily look beyond religious superstition (cf. the folks who helped Salvadorean refugees under Reagan administration in the name of a dead man on a stick) - as long as it is for a good cause. However, I do not know whether the petition in question is for a good or a bad cause, because the site that contains it has very little information about the case.
The fact that someone was refused a visa is quite common in the academia nowadays, and even mainstream publications such as the Johns Hopkins Magazine decry that, as academic instituions lose students and visiting scholars. So even if we assume that the decision in this particular case of Professor Tariq Ramadan was completely arbitrary, why does he deserve a special attention?
Perhaps there is a good reason for that (as in most other similar cases, I presume), but the people who asked us to sign the petition did not divulge it.
PS. I think petition writing is a form of ritualistic behavior that makes people who sign them think that they are doing something without actually doing anything. The recently circulated petition to terrorists who abducted some peace activists is a case in point. The ritualistic behavior used by religious right - tithing - has at leas a vantage point of generating funds for the cause or the crusader, but petition writing not only has a zero effect, but also consumes resources instead of generating them. A total waste of time and energy.
Wojtek