Why no "speaker B" connection on many SS power amp

A few months ago I asked for advice on adding a power amp to my receiver-based HT system here on Audiogon. I received some really helpful feedback. I will continue to power the surrounds with the receiver and add a 3-channel amp for the fronts. However, in my search for an amp for my needs, (solid state, high current, at least 100 "real" watts), I've found hardly any power amps that provide a connection for a second pair of speakers. I currently run a second pair of speakers in the kitchen and I don't want to give that up when I upgrade the power amp. My receiver's "speaker B" output is disabled when you add an external power amp.

The Arcam P35x3 (and Alpha 10x3) does provide "A/B" speaker connections and would be an upgrade, but none of the other really good, more powerful amps that I like provide the "speaker B" hookup:

Aragon 8008X3
McCormack HT3
Sunfire Theater Grand

Why is that? Will it degrade the amp sonically, is it for cost considerations, what do ya'll think?
Simple preconception: Or maybe post-conceptual elitism. Um this is probably on of those questions someone might not want to touch with a twenty-foot pole. But bear-in-mind (get it) that some will offer the convenience, but only in a form consisting of two pairs of speaker outputs, such as might be found on the Musical fidelity amplifiers or something like a Mark Levinson, or even like the Arcams as you mentions. However, only the Arcams which I think sound pretty swell offer such a receiver-like option as a speaker A/B switch. Oops there it is.
You may find this function in flagship or pro grade SS Amps from previous generation. Typical example: Marantz PM-90 and Chord SPM900
A Couple of thoughts.

If your receiver has pre out for additional amplification,
it may have a pre in, or main in, back to the internal
amplifier. If this is the case you can use a high quality
Y jack, and split the left/right pre out and send on pair
to the seperate power amp, the other back into the receiver
utilizing it's internal main amplifier to your kitchen.

Finding spkr B outputs on a multi channel amp is going
to be tough. If you decide to use a seperate speaker
selector look into the Niles MSA10. When you are only
using the main pair of speakers it has decent quality
direct connects (bypasses any impedance matching) from
the amplifier to the speakers.
Thanks for the recommendations above. The 3-channel Arcam unit is looking better and better as the "right tool for the job" for my particular needs (music and HT). It would keep things simple and still be a nice upgrade sonically (I should hope!) and take some of the burden off my receiver's power supply when playing 5.1 surround movies.

Diode - I appreciate your suggestions. Unfortunately, my receiver (Pro-Logic era Sony ES) does not provide for "main-ins" except for the 5.1 analog input which is already being used for my DVD player. It does have a tape monitor loop, but I'm not sure it would work the same as a "pre-out, main-in" loop like you suggested. In fact, in order to use a 3-channel outboard amp for matching power to the front three speakers, you have to engage a feature called "power swap". In this mode, you connect your power amp to the receiver's three preouts and your surround speakers to the main "speaker A" posts (the speaker B is disabled) and only surround signals are played through the receiver's main amps. There is no way to hook up a full 5-channel outboard amp, either. Kind of a unique (weird?) architecture to provide an upgrade path but it limits your options for running an extra pair of stereo speakers for music unless the power amp has the extra hookups. Cheers! JZ