Protest your disaproval of this direct to Enom or notify same through your Enom re-seller.
If one registrar gets away with this it wont be long before all registrars start doing the same thing.
(Register.com has been doing it for Years by adding banners to redirected domain names and you have to pay additional costs to have the banners removed.)
10. "Name Only" Services, Incomplete DNS Information, and Use of Free Services:You agree that if you purchase "Name Only" services or that if you provide incomplete DNS information (the information described in paragraphs 7.iv.E. and 7.iv.F.) in your Account Information, that eNom may direct the domain name to an IP address designated by eNom, including, without limitation, to an IP address which hosts a parking page or a commercial search engine. If you provided incomplete DNS information (and you did not purchase "Name Only" services), you understand that you can update and thereby override the DNS information designated by eNom.
In consideration for providing additional optional services for which eNom does not charge an additional fee, including, but not limited to, URL forwarding, email forwarding, free parking page, free website hosting, free email services, or other services which eNom may introduce from time to time but for which there is not a separate fee ("Free Services"), you agree that, if you use such Free Services, eNom may display advertising in conjunction therewith through the use of pop-up or pop-under browser windows, banner advertisements, audio or video steams, appendices to emails, or other similar advertising means, and that eNom may aggregate related usage data by means of cookies and other similar means.
As Enom seem to operate on fairly small margins, this is simplay an additional way for them to gain revenue. Most other major registrars I'm aware of don't operate in this way, although whether they have similar clauses in their terms & conditions, I've no idea.
Remember that if any banners are proving to be a problem, it's a straightforward task to transfer to another registrar!
R.