I usually see the opposite, where the country, like Russia for example, is unable to keep its currency high, because they don't have enough foreign currency reserves to buy their own national currency.
More prosaically, also, every time the SNB prints a swiss franc, that piece of paper is marked as a liability on its balance sheet (and the corresponding euro amount purchased is the asset). So they optically "lose money" if their assets decline in value versus their liabilities, as happened last week. Of course, they could then print more CHF to cover the loss, but then we're back to my first point.
Finally, and as an aside, the biggest loser in this whole thing was of course the SNB itself, at least on paper. It has CHF liabilities and foreign currency (and gold) assets to the tune of 500bn dollars. On paper then that was a 75bn USD loss. Conversely, and this is worth keeping in mind amidst the frenzy of headlines about broker losses, remember that markets must net off, so all those swiss francs that the SNB sold to defend against appreciation, were held by the market (individuals, brokers, funds, corporates, other governments). Thus it is likely that there are some huge winners out there, whose net wins are greater than the net losses to the tune of about 200bn * 15% (the amount that they intervened in the past 2 years * the percentage move).
Long story short, I don't personally know of any central banks who explicitly promise gold back against their liabilities anymore (this disappeared at Bretton Woods). Thus your claim is (mostly) on the credibility (goodwill) of the bank which is highly dependent on it not issuing too much paper.
As far as your claim on the tangible assets is concerned (the foreign currencies and gold), you will not be able to walk into the SNB with a 100CHF note and ask for the tangible share of what that represents. In practise, if the CHF loses value, the SNB will (amongst other measures such as raising interest rates and trying to put pressure on its government to spend less), protect your 100 CHF by selling its foreign currencies and gold into the market, "defending" your asset. That's how your claim on the tangibles works out in practise: the assets are paid back out to the market if there is pressure on the currency. Of course, again, only a small portion of the value of the CHF will be backed by gold and FX. The rest of the value comes from the fact that it is accepted by merchants and people in exchange for real goods and services. That's goodwill, otherwise known as "credibility" in monetary economics.
In a statement explaining its policy, the SNB points to divergence among major
economies; in particular, the weakening euro which has hit about the lowest
level since its inception.
With anticipated cash injections by the ECB, the euro is expected to depreciate
more against the US dollar and as the franc is pegged to the euro, the
Swiss franc is weakening versus the dollar too.
So, they conclude that there is no longer "exceptional overvaluation" of the
Swiss franc that justified the minimum exchange rate.They likely have a policy against printing Francs in arbitrary amounts to not endanger the long-term trust in the currency.