IME this is common practice. Every major job I've had done (windows/extension/fencing etc) has involved the companies doing the work sticking their advertising on the front of the house (no front fencing).
My only problem here would be if they have actually damaged the fence - in which case I'd be looking for them to make good the damage or give a discount on your job.
Well they've been working for me since Monday and they only put the sign up yesterday when I was out of the house all day, they're not working today but are finishing off Friday, so I 'll ask them to remove it. Just don't want to piss them off whilst they have my door keys. I think it would have been polite if they'de have asked.
It doesn't matter if it's common practice or not, it's good manners to at least mention it first - not everyone wants to have it attached to their fence. YANBU at all (or petty) but communicate with them as they should have done with you!!