Why are offers restricted to certain advertising mediums?

[tubedogg]

I see a lot of offers that have terms and conditions such as:

• "Must be promoted via email only"
• "Can not be promoted via: paid search"
• ... etc

Why do such clauses exist (ie: why do they care where their money comes from).
I'm guessing that a penalty for violating is a swift boot?

 

[mkrongel]

certain sources of traffic convert better for the advertiser than others.

Or they do their own search in the cases of no search.

 

[DangerBrown]

It may also be that an advertiser has found that certain types of traffic are more likely to file for chargebacks, or that they don't have unlimited finances and plan to slowly open new traffic sources as they can afford to.