Fair enough and a good point I hadn't considered.
On further reflection, it seems like a computer operating in something resembling a infrastructure mode client would have little reason to transmit until it saw something that gave it an idea of what frequencies and powers were appropriate.
And, since I'm actually allergic to too much idle speculation[*] without a good dose of raw facts, I actually had a go at looking up the relevant standard. Indeed, the actual default is "passive scanning" (highlighted below):
9.9.1 Operation upon entering a regulatory domain
A STA that is enabled for operation across regulatory domains shall default to passive scanning when it has lost connectivity with its ESS. Passive scanning is performed using only the receive capabilities of the station and is, thus, compatible with regulatory requirements. The timeout for determining the loss of connectivity is system dependent and beyond the scope of this standard.
When a STA enters a regulatory domain, it shall passively scan to learn at least one valid channel, i.e., a channel upon which it detects IEEE Std 802.11 frames. The Beacon frame contains information on the country code, the maximum allowable transmit power, and the channels to be used for the regulatory domain. Optionally, the Beacon frame may also include, on a periodic basis, the regulatory information that would be returned in a Probe Response frame. Once the STA has acquired the information so that it is able to meet the transmit requirements of the regulatory domain, it shall transmit a Probe Request to an AP to gain the additional regulatory domain information contained in the Probe Response frame, unless the information was previously received in a Beacon fame. The STA then has sufficient information available to configure its PHY for operation in the regulatory domain.
(Quoted from here.)
So, I guess that's the ultimate way to not break any transmit laws... transmit nothing until you hear a valid local transmission.
[*] That's not to say I haven't been guilty of such in the past, nor does it say I won't be guilty of it in the future. ;-)
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