Thanks for all the responses. Really an eye-opener for me.
Had the meeting with the hotel, and apparently they don't have an in-house vendor, the hotel owns the AV. And this policy is not a blanket policy on all events. They're not really bothered about small events, but rather larger events, where a sub-standard outside vendor affects the image of the hotel. They say this 20% is to cover them having to provide drapes, skirting for control, even uniforms for outside vendors.
And as I brought up, this charge should be negotiated between the client & them. Different story if we were dealing with the hotel on behalf of a client. Of course, if the hotel refers a guest to us, we have to mark-up 20% for them. That's a sales commission, no objection there.
At the end of the day, we agreed to take this forward on a case-by-case basis, so I guess we're good so far. Let's see how this develops over time.
sheldon