It's like "those people told me that they are free, but I cannot sell them, so in reality they are non-free".
Really about the only scenario I can think of where I'd reasonably expect to hit this case is if I were tasked with building the software stack for some kind of appliance that would be greatly simplified by throwing Linux on there. And even then, the custom software running as a process on the box would presumably not have to be Copyleft-licensed, just any changes to the Kernel or any other bundled GPL'd software would have to be made available (if in fact there are any changes), and if GPLv3 is involved then the box would have to have some way for the user to replace the software (but the Linux Kernel is GPLv2 so that may not be an issue).
Barring that scenario, if your idea is "I'll develop something for customer#1 and slap the GPL license on it, then customer#1 has no choice but to release the source and I can reuse it for customer#2", unfortunately that's incorrect. Work done for hire is owned by the employer, not the employee, so you can't slap the GPL license on work you do for customer#1 unless customer#1 authorizes it. And in fact if the entire work is built in-house (rather than pulling in GPL'd libraries), then it doesn't even matter if you claim that it's GPL'd because the customer would be the sole copyright owner and would be able to relicense it. The only reason they couldn't relicense it is if it pulls in third-party GPL'd code or libraries, but if I were customer#1 and I hired a developer who pulled in GPL'd libraries without permission in order to try and force the entire work to be released as open source, you can bet I'd be firing that developer on the spot.
No, I cannot. Work done for hire is owned by the employer, not the employee. But, if employer chose GPL-covered software, e.g. Linux kernel, then he is forced to release code under GPL, so I can develop my code, e.g. kernel module for customer #1 and then use it with customer#2, because GPL protects my freedom. I don't need to reinvent the wheel, like with proprietary code.
"Customer" implies a much looser relationship, but you should still make sure appropriate contracts defining ownership are in place.
Tangential rant: "Copyleft" licenses aren't actually free. The GNU claims they are, that the restrictions that copyleft licenses add don't conflict with the "central freedoms" but simply protects them. But they do restrict my freedom. If a piece of source was truly free, I should be allowed to make significant modifications and sell the resulting software (without giving out the source). Copyleft expressly prohibits this. I get that the GNU is dreaming of a utopia in which all software is open source and "free" and nobody ever makes closed-source software or charges for software. But we don't live in that utopia, and we probably never will; the closest we'll come is when we hit a post-scarcity society and do away with money, so no more keeping software closed-source in order to sell it, but there will still be reasons to keep software closed-source other than wanting to charge money for it. But I digress. The point is, Copyleft licenses rather significantly restrict my freedom as a developer. I'm not trying to debate as to whether or not that restriction is a good thing, just taking issue with calling the result "freedom".