Both of those jobs require a large investment of time, and yet he's expected to make hundreds of seals for the village every month?
I believe that it was specified that Jiraiya was exempt from this, due to serving the village exactly by being the Hokage.
Also, regarding the tax not making sense, here's a possible explanation:
Leaf wants sealmasters, yes. But they primarily want sealmasters because they want
seals for their ninja. Now, if they didn't have something like this, what might happen might be that the clans basically decided that they'd rather have
their sealmasters work on super-secret projects and then claim "clan secrets" and say that no, the rest of the village aren't allowed these new tools, or only if they pay exorbitant prices for them, and a bunch of other conditions. Oh, and also produce things like explosive seals, but only for intra-clan trading, which neither the MC nor the Hokage would be likely to mess with.
What
would then happen would be that there'd be a hole in the market, and somebody would of course try to fill it out. Either a clan wanting money, or just a clanless ninja actually managing to become a sealmaster, and deciding that hey, spending every hours or the day making explosive or storage seals isn't that bad, when you can live nicely for it.
However, I imagine that most sealmasters would rather not do tasks like making explosive tags, probably considering it a waste of their talents, and would much rather try to make something that comes close to the sheer awesomeness of our inventions.
So let's say that of the 30 sealmasters we have in Leaf... let's say that two thirds of these would be clan ninja. Of these twenty, let's say that fifteen of those are influential enough to tell the clan that they'll do a bare minimum of "menial sealwork", but no more. Five would be more junior sealmasters, and be willing/bullied into making lots of "simple" seals - enough to exceed the clan's internal demands - and then sold out on the open market.
Now, let's look at the ten clanless sealmasters. I imagine that half would fall into the category of "screw you, I want to research, not make seals that I could make in my sleep", as I imagine that clanless ninja who manage to become sealmasters have some measure of financial independence already, so some of them would rather spend their time doing interesting research, and maybe occasionally making some "menial" seals and selling them if they're in a financial pinch.
For the remaining five, let's say that two are affiliated with organizations that have significant internal demands of their own - ANBU, possibly T&I - which would leave us with just three clanless sealmasters who reliably sells on the open market.
That way, the entire need for "standard" seals among the ninja forces would have to be met by something like eight to ten sealmasters. However, this would a) mean that the clans would, if they wanted, be able to hike the prices significantly, simply by withholding their own seals, b) disadvantage clanless ninja even more, as there are said to be clans without sealmasters, who would likely be willing to shell out significantly more than the average clanless ninja, c) possibly cause market to fluctuate severely, as eight to ten people could easily mean that some act of god happenstance might see several of them incapacitated, or end up producing more/less than their average, and d) actively encourage the clans actually selling to the open market to wage economic warfare against the clanless sealmasters.
There's probably more points that could be made by following this train of thought, as well as plenty of holes, but it's getting a bit late for me, so I'll just leave it here.
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